High Performance Web Sites blog

I recently hosted Andy Hopper (University of Cambridge) at Google to deliver his talk on Computing for the Future of the Planet (YouTube video). He covered a variety of areas that ranged from practical deployment issues that should be addressed today to futuristic concepts. The beginning of the talk, the part that was of most interest to me, was about energy efficient computing: adaptive datacentres and a “virtual battery” (moving computing tasks to where energy is available). Power consumption is certainly a key focus for Google and other large web companies. It’s intriguing to think about shifting data and processes in real time based on the cost of energy.

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Today Dion Almaer announced the Google AJAX Libraries API. This is a great resource for developers using any of the popular JavaScript frameworks including Prototype, Script.aculo.us, jQuery, Dojo, and MooTools. Rather than downloading it to your own server and hosting it from there, you can request your preferred JavaScript library from ajax.googleapis.com:

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.2.6/jquery.min.js">
</script>

The greatest benefit in my opinion is that any developer can leverage Google’s CDN to deliver the bulk of their JavaScript. From YSlow and my book I received a lot of feedback that Rule 2: Use a CDN was out of reach for many developers. I use jQuery in one of my personal projects and serve it on my hosted web site from one geographic location. Ouch. Being able to move that 21K closer to my users is great.

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[This post is based on a chapter from my next book, a follow-up to High Performance Web Sites containing more best practices for faster web sites.]

The growing adoption of Ajax and DHTML means today’s web pages have more JavaScript than ever before. The average top ten U.S. web site[1] contains 252K of JavaScript. JavaScript slows pages down. When the browser starts downloading or executing JavaScript it won’t start any other parallel downloads. Also, anything below an external script is not rendered until the script is completely downloaded and executed. Even in the case where external scripts are cached the execution time can still slow down the user experience and thwart progressive rendering.

Every line of JavaScript code matters to a fast loading page. And yet, as shown in Table 1, the average top ten U.S. web site only executes 25% of the JavaScript functionality before the onload event.[2] Why track this relative to the onload event? Any code needed after the onload event (for dropdown menus, Ajax requests, etc.) could be downloaded later, making the initial page render more quickly.

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