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.
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!
YSlow analyzes Web pages and tells you why they're slow based on Yahoo's rules for high performance Web sites. YSlow gives you performance report card, HTTP or HTML summary, list of components in the page, and tools including JSLint.