Technology

Best Practices for Speeding up Your Web Site

The Exceptional Performance team has been famous a number of times in best practices for building web pages fast. The list includes 34 best practices divided into 7 categories.

Minimize HTTP Requests

Tag: content

80% of the end-user reaction time is spent on the front-end. Most of this time is tied up in downloading all the workings in the page: images, style sheets, script, flash, etc. Reducing the number of works in turn reduces the number of HTTP requests essential to render the page. This is the key to faster pages.

One way to decrease the number of components in the page is to shorten the page's design. But is there a way to build pages with richer content while also getting fast response times? Here are some techniques for decreasing the number of HTTP requests, while still supporting rich page designs.

United files are a way to decrease the number of HTTP requests by joining all scripts into a single script, and similarly joining all CSS into a single style sheet. Combining files is more challenging when the scripts and style sheets differ from page to page, but building this part of your release method improves response times.

CSS Sprites are the preferred method for reducing the number of image requests. Join your background images into a single image and use the CSS background-image and background-position properties to display the preferred image segment.

Image maps join multiple images into a single image. The overall size is about the same, but decreasing the number of HTTP needs speed up the page. Image maps only work if the images are neighboring in the page, such as a navigation bar. Defining the coordinates of image maps can be boring and error prone. Using image maps for navigation is not accessible too, so it's not recommended.

Inline images use the data: URL scheme to set in the image data in the actual page. This can add to the size of your HTML document. Joining inline images into your (cached) style sheets is a way to decrease HTTP requests and keep away from increasing the size of your pages. Inline images are not yet supported across all major browsers.

Decreasing the number of HTTP requests in your page is the place to start. This is the most significant guideline for improving performance for first time visitors. As described in Tenni Theurer's blog post Browser Cache Usage – Exposed, 40-60% of daily visitors to your site come in with an empty store. Creating your page fast for these first time visitors is to a better user experience.

Use a Content Delivery Network

Tag: server

The user's closeness to your web server has an impact on reply times. Deploying your content across multiple, geographically isolated servers will create your pages load faster from the user's point of view. But where must you start?

As a first step to implementing geographically isolated content, don't attempt to redesign your web application to work in a distributed architecture. Depending on the application, changing the structural design could comprise of daunting tasks such as synchronizing session state and replicating database dealings across server locations. Attempts to reduce the distance between users and your content could be delayed by, or never pass, this application structural design step.

Remember that 80-90% of the end-user response time is spent downloading all the workings in the page: images, style sheets, scripts, flash, etc. This is the Performance Golden Rule. Rather than starting with the complex task of redesigning your application structural design, it's better to first disperse your static content. This not only gets a bigger reduction in response times, but it's easier thanks to content delivery networks.

A content delivery network (CDN) is a collection of web servers spread across multiple locations to deliver content more powerfully to users. The server chosen for delivering content to a particular user is typically based on a measure of network closeness. For example, the server with the smallest number of network hops or the server with the quickest reply time is selected.

Some large Internet companies own their own CDN, but it's gainful to use a CDN service provider, such as Akamai Technologies, Mirror Image Internet, or Limelight Networks. For start-up companies and private web sites, the cost of a CDN service can be too expensive, but as your target audience grows larger and becomes more global, a CDN is essential to get fast response times. At Yahoo!, properties that moved fixed content off their application web servers to a CDN improved end-user response times by 20% or more. Moving to a CDN is a relatively easy code change that will dramatically get better the speed of your web site.

Add an Expires or a Cache-Control Header
Tag: server

There are two things in this rule:

  • For still components: implement "Never expire" policy by setting far future Expires header
  • For energetic components: use an suitable Cache-Control header to help the browser with conditional requests

Web page designs are receiving richer and richer, which means more scripts, style sheets, images, and flash in the page. A first-time visitor to your page may have to create many HTTP requests, but by using the Expires header you create those components cacheable. This avoids unnecessary HTTP requests on subsequent page views. Expires headers are most often used with images, but they must be used on all workings comprising of scripts, style sheets, and flash components.

Browsers (and proxies) use a store to decrease the number and size of HTTP requests, creating web pages load faster. A web server uses the Expires header in the HTTP answers to tell the client how long a component can be cached. This is a far future Expires header, telling the browser that this reply won't be stale until April 15, 2010.
     
Expires: Thu, 15 Apr 2010 20:00:00 GMT

If your server is Apache, use the Expires Default command to set an ending date relative to the present date. This example of the Expires Default command sets the Expires date 10 years out from the time of the request.

Expires Default "access plus 10 years"

Keep in mind, if you use a far future Expires header you have to change the component's filename whenever the component changes. At Yahoo! we often create this step part of the build course: a version number is embedded in the component's filename, for example, yahoo_2.0.6.js.

Using a far future Expires header affects page views only after a user has already visited your site. It has no effect on the number of HTTP requests when a user visits your site for the first time and the browser's cache is empty. Therefore the impact of this performance improvement depends on how often users hit your pages with a primed cache. (A "primed cache" already contains all of the components in the page.) We measured this at Yahoo! and found the number of page views with a ready cache is 75-85%. By using a far future Expires header, you increase the number of workings that are cached by the browser and re-used on following page views without sending a single byte over the user's Internet connection.

Gzip Components
Tag: server

The time it takes to move an HTTP request and reply across the network can be considerably reduced by decisions made by front-end engineers. It's true that the end-user's bandwidth speed, Internet service provider, proximity to peering exchange points, etc. are beyond the control of the development team. But there are other variables that affect reply times. Density reduces response times by reducing the size of the HTTP reply.

Starting with HTTP/1.1, web clients indicate support for compression with the Accept-Encoding header in the HTTP request.
Accept-Encoding: gzip, deflate

If the web server sees this header in the request, it may squeeze the response using one of the methods listed by the client. The web server notifies the web client of this via the Content-Encoding header in the reply.

Content-Encoding: gzip

Gzip is the most established and effective compression method at this time. It was developed by the GNU project and standardized by RFC 1952. The only other compression format you're likely to see is reduce, but it's less effective and less popular.

Gzipping usually reduces the response size by about 70%. Just about 90% of today's Internet traffic travels through browsers that claim to support gzip. If you use Apache, the module configuring gzip depends on your version: Apache 1.3 uses mod_gzip while Apache 2.x uses mod_deflate.

There are known issues with browsers and proxies that may cause a mismatch in what the browser expects and what it receives with regard to compressed content. Fortunately, these edge cases are dwindling as the use of older browsers drops off. The Apache modules help out by adding apt Vary response headers automatically.

Servers opt what to gzip based on file type, but are typically too limited in what they choose to compress. Most web sites gzip their HTML documents. It's also worthwhile to gzip your scripts and style sheets, but several web sites miss this opportunity. In fact, it's worthwhile to compress any text response including XML and JSON. Image and PDF files must not be gzipped because they are already compressed. Trying to gzip them not only wastes CPU but can potentially increase file sizes.

Gzipping as several file types as possible is an easy way to decrease page weight and speed up the user experience.

Put Style sheets at the Top
Tag: css

While researching performance at Yahoo!, we discovered that moving style sheets to the document HEAD makes pages appear to be loading faster. This is because putting style sheets in the HEAD permits the page to render increasingly.

Front-end engineers that care about performance desires a page to load progressively; that is, we want the browser to display whatever content it has as soon as possible. This is chiefly significant for pages with a lot of content and for users on slower Internet connections. The significance of giving users visual feedback, such as progress indicators, has been well researched and documented. In our case the HTML page is the progress indicator! When the browser loads the page progressively the header, the navigation bar, the logo at the top, etc. all serve as visual feedback for the user who is waiting for the page. This improves the overall user experience.

The problem with putting style sheets near the bottom of the document is that it prohibits progressive rendering in many browsers, comprising of Internet Explorer. These browsers block rendering to avoid having to redraw elements of the page if their styles change. The user is stuck viewing a blank white page.

The HTML specification clearly states that style sheets are to be included in the HEAD of the page: "Unlike A, [LINK] may only appear in the HEAD section of a document, although it may appear any number of times." Neither of the alternatives, the blank white screen or flash of unsettled content, are worth the risk. The optimal solution is to follow the HTML requirement and load your style sheets in the document HEAD.

Put Scripts at the Bottom
Tag: JavaScript

The problem caused by scripts is that they block parallel downloads. The HTTP/1.1 specification suggests that browsers download no more than two components in parallel per hostname. If you serve your images from multiple hostnames, you can get more than two downloads to occur in parallel. While a script is downloading, however, the browser won't start any other downloads, even on different hostnames.

In some situations it's not easy to move scripts to the bottom. If, for example, the script uses document.write to insert part of the page's content, it can't be moved lower in the page. There may also be scoping issues. In several cases, there are ways to workaround these situations.

An alternative suggestion that often comes up is to use delayed scripts. The DEFER attribute indicates that the script does not contain document.write, and is a clue to browsers that they can continue rendering.  Unluckily, Firefox doesn't support the DEFER attribute. In Internet Explorer, the writing may be deferred, but not as much as desired. If a script can be deferred, it can also be moved to the bottom of the page. That will make your web pages load faster.

Avoid CSS Expressions
Tag: css

CSS terminology is a powerful (and dangerous) way to set CSS properties animatedly. They're supported in Internet Explorer, starting with version 5. As an example, the background color could be set to exchange every hour using CSS expressions.

background-color: expression ((new Date ()).getHours() %2? "#B8D4FF”: "#F08A00”);

As shown here, the expression method accepts a JavaScript expression. The CSS property is set to the result of evaluating the JavaScript expression. The expression method is unseen by other browsers, so it is useful for setting properties in Internet Explorer required to make a consistent experience across browsers.

The problem with expressions is that they are evaluated more often than most people expect. Not only are they evaluated when the page is rendered and resized, but also when the page is scrolled and even when the user moves the mouse over the page. Adding a counter to the CSS expression permits us to keep track of when and how often a CSS expression is evaluated. Moving the mouse around the page can easily generate more than 10,000 evaluations.

One way to decrease the number of times your CSS expression is evaluated is to use one-time expressions, where the first time the expression is evaluated it sets the style property to an open value, which replaces the CSS expression. If the style property should be set animatedly throughout the life of the page, using event handlers instead of CSS expressions is a choice comes up to. If you must use CSS expressions, remember that they may be evaluated thousands of times and could affect the performance of your page.

Make JavaScript and CSS External
Tag: JavaScript, css

Several of these performance rules deal with how outdoor components are managed. However, before these considerations arise you must ask a more basic question: Should JavaScript and CSS are contained in external files, or inlined in the page itself?

Using external files in the true world usually produces faster pages because the JavaScript and CSS files are cached by the browser. JavaScript and CSS that are inlined in HTML documents get downloaded every time the HTML document is requested. This decreases the number of HTTP requests that are required, but increases the size of the HTML document. On the other hand, if the JavaScript and CSS are in external files cached by the browser, the size of the HTML document is decreased without increasing the number of HTTP requests.

The key factor, then, is the frequency with which outdoor JavaScript and CSS components are cached relative to the number of HTML documents requested. This factor, although hard to quantify, can be gauged using a variety of metrics. If users on your site have multiple page views per session and several of your pages re-use the same scripts and style sheets, there is a greater potential advantage from cached external files.

Several web sites fall in the middle of these metrics. For these sites, the best solution usually is to deploy the JavaScript and CSS as external files. The only omission where inlining is preferable is with home pages, such as Yahoo!'s front page and My Yahoo! Home pages that have few (perhaps only one) page view per session may find that inlining JavaScript and CSS results in faster end-user reply times.

For front pages that are classically the first of several page views, there are techniques that leverage the reduction of HTTP requests that inlining gives, as well as the caching profit got through using external files. One such technique is to inline JavaScript and CSS in the front page, but energetically downloads the external files after the page has finished loading. Subsequent pages would reference the external files that must already be in the browser's cache.

Reduce DNS Lookups
Tag: content

The Domain Name System (DNS) maps hostnames to IP addresses, just as phonebooks map people's names to their phone numbers. When you type www.yahoo.com into your browser, a DNS resolver contacted by the browser returns that server's IP address. DNS has a cost. It classically takes 20-120 milliseconds for DNS to lookup the IP address for a given hostname. The browser can't download anything from this hostname until the DNS lookup is finished.

DNS lookups are cached for better presentation. This caching can occur on a special caching server, maintained by the user's ISP or local area network, but there is also caching that occurs on the individual user's computer. The DNS information remains in the operating system's DNS cache (the "DNS Client service" on Microsoft Windows). Most browsers have their own caches, private from the operating system's cache. As long as the browser keeps a DNS record in its own cache, it doesn't bother the operating system with a demand for the record.

Internet Explorer caches DNS lookups for 30 minutes by default, as particular by the DnsCacheTimeout registry setting. Firefox caches DNS lookups for 1 minute, controlled by the network.dnsCacheExpiration design setting. (Fasterfox changes this to 1 hour.)

When the client's DNS cache is empty (for both the browser and the operating system), the number of DNS lookups is equal to the number of rare hostnames in the web page. This comprises of the hostnames used in the page's URL, images, script files, style sheets, Flash objects, etc. Decreasing the number of rare hostnames decreases the number of DNS lookups.

Decreasing the number of rare hostnames has the potential to decrease the amount of parallel downloading that takes place in the page. Avoiding DNS lookups cuts response times, but decreasing parallel downloads may increase response times. My guideline is to split these components across at least two but no more than four hostnames. This result in a good compromise between decreasing DNS lookups and permitting a high degree of parallel downloads.

Minify JavaScript and CSS
Tag: JavaScript, css

Minification is the practice of removing needless font from code to decrease its size thereby improving load times. When code is minified all comments are removed, as well as unnecessary white space font (space, newline, and tab). In the case of JavaScript, this improves response time performance because the size of the downloaded file is reduced. Two popular tools for minifying JavaScript code are JSMin and YUI Compressor. The YUI compressor can also minify CSS.

Obfuscation is a choice optimization that can be useful to source code. It's more complex than minification and thus more likely to make bugs as a result of the obfuscation step itself. In a survey of ten top U.S. web sites, minification achieved a 21% size decrease versus 25% for obfuscation. Although obfuscation has a higher size reduction, minifying JavaScript is less risky.

In addition to minifying external scripts and styles, inlined
An alternative in PHP would be to generate a function called insertScript.
  

In addition to preventing the same script from being inserted multiple times, this function could handle other issues with scripts, such as need checking and adding version numbers to script filenames to support far future Expires headers.

Configure ETags
Tag: server

Entity tags (ETags) are a device that web servers and browsers use to decide whether the component in the browser's cache matches the one on the origin server. (An "entity" is another word a "component": images, scripts, style sheets, etc.) ETags were added to give a mechanism for validating entities that is more flexible than the last-modified date. An ETag is a cord that exclusively identifies a particular version of a component. The only format constraints are that the string be quoted. The origin server specifies the component's ETag using the ETag reply header.
     
HTTP/1.1 200 OK
Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT
ETag: "10c24bc-4ab-457e1c1f"
Content-Length: 12195

Later, if the browser has to confirm a component, it uses the If-None-Match header to pass the ETag back to the basis server. If the ETags match, a 304 status code is returned dropping the response by 12195 bytes for this example.
     
GET /i/yahoo.gif HTTP/1.1
Host: us.yimg.com
If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT
If-None-Match: "10c24bc-4ab-457e1c1f"
HTTP/1.1 304 Not Modified

The difficulty with ETags is that they classically are constructed using attributes that make them rare to a particular server hosting a site. ETags won't match when a browser gets the creative component from one server and later tries to confirm that component on a different server, a situation that is all too common on Web sites that use a cluster of servers to handle requests. By default, both Apache and IIS embed data in the ETag that dramatically reduces the odds of the validity test ensuing on web sites with multiple servers.

The ETag arrangement for Apache 1.3 and 2.x is inode-size-timestamp. Although a given file may reside in the same directory across multiple servers, and have the same file size, permissions, timestamp, etc., its inode is dissimilar from one server to the next.

IIS 5.0 and 6.0 have a similar problem with ETags. The arrangement for ETags on IIS is Filetimestamp: ChangeNumber. A ChangeNumber is an answer used to track configuration changes to IIS. It's unlikely that the ChangeNumber is the same across all IIS servers behind a web site.

The end result is ETags generated by Apache and IIS for the exact same component won't match from one server to another. If the ETags don't match, the user doesn't get the small, fast 304 response that ETags were designed for; instead, they'll get a normal 200 reply along with all the data for the component. If you host your web site on just one server, this isn't a problem. But if you have multiple servers hosting your web site and you're using Apache or IIS with the default ETag configuration, your users are getting slower pages, your servers have a higher load, you're consuming greater bandwidth, and proxies aren't caching your content efficiently. Even if your components have a far future Expires header, a conditional GET request is still made whenever the user hits Reload or Refresh.

If you're not taking benefit of the flexible validation model that ETags give, it's better to just remove the ETag altogether. The Last-Modified header validates based on the component's timestamp. And removing the ETag decreases the size of the HTTP headers in both the response and subsequent requests. This Microsoft Support article describes how to remove ETags. In Apache, this is done by simply adding the following line to your Apache configuration file:
     
FileETag none
Make Ajax Cacheable
Tag: content

One of the cited profits of Ajax is that it gives instantaneous feedback to the user because it needs information asynchronously from the backend web server. However, using Ajax is no guarantee that the user won't be twiddling his thumbs waiting for those asynchronous JavaScript and XML responses to return. In several applications, whether or not the user is kept waiting depends on how Ajax is used. For example, in a web-based email client the user will be kept waiting for the results of an Ajax request to find all the email messages that match their search criteria. It's significant to remember that "asynchronous" does not imply "instantaneous".

To get better presentation, it's significant to optimize these Ajax responses. The most significant way to get better the presentation of Ajax is to create the responses cacheable, as discussed in Add an Expires or a Cache-Control Header. Some of the other rules also apply to Ajax:

  • Gzip Components
  • Reduce DNS Lookups
  • Minify JavaScript
  • Avoid Redirects
  • Configure ETags

Let's look at a case. A Web 2.0 email client might use Ajax to download the user's address book for auto completion. If the user hasn't adapted her address book since the last time she used the email web app, the previous address book response could be read from cache if that Ajax response was made cacheable with a future Expires or Cache-Control header. The browser should be informed when to use a previously cached address book answer versus requesting a new one. This could be done by adding a timestamp to the address book Ajax URL representing the last time the user modified her address book, for example, &t=1190241612. If the address book hasn't been customized since the last download, the timestamp will be the same and the address book will be read from the browser's cache eliminating an extra HTTP roundtrip. If the user has modified her address book, the timestamp make sure the new URL doesn't match the cached answers, and the browser will request the updated address book entries.

Even though your Ajax responses are shaped energetically, and might only be applicable to a single user, they can still be cached. Doing so will create your Web 2.0 apps faster.

Flush the Buffer Early
Tag: server

When users ask for a page, it can take anywhere from 200 to 500ms for the backend server to stitch together the HTML page. During this time, the browser is at rest as it waits for the data to enter. In PHP you have the function flush(). It allows you to send your incompletely ready HTML answer to the browser so that the browser can start fetching components while your backend is busy with the rest of the HTML page. The advantage is mainly seen on busy backends or light frontends.

A good place to think flushing is right after the HEAD because the HTML for the head is generally easier to produce and it permits you to comprise any CSS and JavaScript files for the browser to start fetching in parallel while the backend is still giving out.

Example:
...

 

...
Yahoo! search pioneered research and actual user testing to prove the advantages of using this technique.

Use GET for AJAX Requests
Tag: server

The Yahoo! Mail team found that when using XMLHttpRequest, POST is implemented in the browsers as a two-step course: sending the headers first, then sending data. So it's best to use GET, which only takes one TCP packet to send (unless you have a lot of cookies). The maximum URL length in IE is 2K, so if you send more than 2K data you may not be able to use GET.

An interesting side affect is that POST without essentially posting any data behaves like GET. Based on the HTTP specs, GET is meant for retrieving information, so it creates sense (semantically) to use GET when you're only requesting data, as opposed to sending data to be stored server-side.

Post-load Components
Tag: content

You can take a closer look at your page and ask yourself: "What's extremely essential in order to render the page originally?” The rest of the content and components can wait.

JavaScript is an ideal applicant for splitting before and after the on load event. For example if you have JavaScript code and libraries that do drag and drop and animations, those can wait, because dragging basics on the page comes after the original rendering. Other places to look for candidates for post-loading comprise of hidden content (content that appears after a user action) and images below the fold.

Tools to help you out in your effort: YUI Image Loader permits you to delay images below the fold and the YUI Get utility is an easy way to comprise of JS and CSS on the fly. For an example in the wild take a look at Yahoo! Home Page with Firebug's Net Panel turned on.

It's good when the performance goals are inline with other web development best practices. In this case, the idea of progressive enhancement tells us that JavaScript, when supported, can improve the user experience but you have to make sure the page works even without JavaScript. So after you've made sure the page works fine, you can enhance it with some post-loaded scripts that give you more bells and whistles such as drag and drop and animations.

Preload Components
Tag: content

Preload may look like the opposite of post-load, but it really has a dissimilar goal. By preloading components you can take benefit of the time the browser is idle and asks for components (like images, styles and scripts) you'll require in the future. This way when the user visits the next page, you could have most of the components already in the cache and your page will load much faster for the user.

There are really many kinds of preloading:

  • Unconditional preload - as soon as on load fires, you go ahead and fetch some additional components. Check google.com for an example of how a sprite image is requested on load. This elf image is not needed on the google.com homepage, but it is needed on the successive search result page.
  • Conditional preload - based on a user action you create an educated guess where the user is headed next and preload accordingly. On search.yahoo.com you can see how some additional components are requested after you start typing in the input box.
  • Anticipated preload - preload in move ahead before launching a redesign. It often happens after a redesign that you hear: "The new site is cool, but it's slower than before". Part of the problem could be that the users were visiting your old site with a full cache, but the new one is always an empty cache experience. You can moderate this side effect by preloading some components before you even launched the redesign. Your old site can use the time the browser is idle and ask for images and scripts that will be used by the new site.

Reduce the Number of DOM Elements
tag: content

A difficult page means more bytes to download and it also means slower DOM access in JavaScript. It creates dissimilarity if you loop through 500 or 5000 DOM elements on the page when you want to add an event handler for example.

A high number of DOM elements can be a symptom that there's something that should be improved with the markup of the page without necessity of removing content. Are you using nested tables for layout purposes? Are you throwing in more

s only to fix layout issues? Maybe there's a better and more semantically correct way to do your markup.

 

A great help with layouts are the YUI CSS utilities: grids.css can help you with the overall layout, fonts.css and reset.css can help you strip away the browser's defaults formatting. This is a opportunity to start fresh and think about your markup, for example use

s only when it makes sense semantically, and not because it renders a new line.

 

The number of DOM elements is easy to test, just type in Firebug's console:
document.getElementsByTagName ('*').length

And how many DOM elements are too many? Check other similar pages that have good markup. For example the Yahoo! Home Page is a pretty busy page and still under 700 elements (HTML tags).

Split Components across Domains
Tag: content

Splitting components permits you to maximize parallel download. Ensure you're using not more than 2-4 domains because of the DNS lookup fine. For example, you can host your HTML and lively content on www.example.org and split static components between static1.example.org and static2.example.org

Minimize the Number of iframes
Tag: content

Iframes permit an HTML document to be inserted in the parent document. It's significant to understand how iframes work so they can be used successfully.

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