Why slow? Now you know.

Uncover problems with Web sites with this free Firefox extension.

YSlow is a question with an answer: "Why slow?" This free Firefox extension can tell you why, and in great detail, when the question happens to be about Web page performance. YSlow tests and grades Web sites based on high-performance criteria established by Yahoo, and issues a report card describing areas for improvement. It also comes with a bag of tools for fixing some of the problems it uncovers.

YSlow is designed to integrate with Firebug, a suite of development tools bundled into a free Firefox extension, and YSlow's developer recommends installing Firebug first. YSlow placed a labeled speed meter icon next to the Firebug icon in the Firefox extension panel. We clicked the icon, and Firebug opened with the YSlow tab displayed. At the same time, YSlow ran its tests and displayed a letter grade and speed statistics in the extension panel. The YSlow tab explains how the app works and offers a check box for the option to test sites automatically when they're loading. Since we used YSlow with Firebug, we chose not to select this option. We browsed to a national news site and clicked the Run Test icon. YSlow ran performance tests and displayed results for 23 categories, with six filter options: Content, Cookie, CSS, Images, JavaScript, and Server. The site received an F letter grade. YSlow offered a lot of advice on what to do to fix the issue on its four tabs: Grade, Components, Statistics, and Tools. We ran YSlow on some other sites--YouTube's home page got a B, while Hulu got a C. Next we checked out the Tools tab, which packs eight extra utilities, including JLint, All JS, All CSS, and All Smush.it. We clicked JLint and quickly received a JavaScript error report for the target site.

We also tried YSlow on personal Web pages, and it revealed some unsuspected problems. Most people with Web sites to manage will want YSlow to analyze those sites, especially when using it with Firebug. That doesn't stop you from running it just to see why any site is so slow to load, though!

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