JavaScript - the Web site performance killer, Google guru says

Nowadays, even regular Web surfers know some of the things to avoid when designing a Web site for fast performance. Cut the number of requests to the Web server. Shrink JPEG sizes. Employ a content delivery network vendor like Akamai Technologies Inc. or Limelight Networks Inc.

Problem is, according to Steve Souders, steps like these aimed at optimizing the Web server make only a tiny impact.

"We used to tear apart the Apache [Web server] code to figure out what Yahoo was doing," said Souders, who was Yahoo Inc.'s chief performance engineer for several years before moving to Google Inc. in the same role.

But after performing a detailed analysis, Souders discovered something startling: Only 10% to 20% of the time it took to load a Web site could be attributed to the Web server.

The vast majority was the result of code executing inside the Web browser, said Souders at a talk on Tuesday at Microsoft Corp.'s Tech Ed conference in Los Angeles (download PowerPoint here).

In today's Ajax-heavy Web sites, the offending code is usually JavaScript, Souders said. That's not because JavaScript files on a Web page are large, they aren't, he said, but because of the way Web browsers treat JavaScript.

"The first generation of Web browsers decided that because they had to execute all of the JavaScript files in order, we might as well execute one while stopping all other downloads," he said, as well as preventing any other code from being executed or rendered.

That may have made sense a decade ago, but in today's era of PCs powered by dual and quad-core CPUs, it doesn't. And the cost of the delays created can be high.

Google has found that a 500-millisecond delay results in a 20% decrease in Web traffic, while Amazon.com has seen a 100 millisecond delay cutting its sales by 1%, Souders said.

Better browsers, better performance

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