YSlow analyzes web pages and tells you why they're slow based on the rules for high performance web sites. YSlow analyzes web page performance and tells you why performance is slow. It is integrated into the Firebug web development tool for Firefox.
YSlow is a Firefox add-on integrated with the popular Firebug web development tool.
How does YSlow work?
YSlow makes three passes to generate its results.
In the first pass, YSlow crawls the DOM to find all the components (images, scripts, stylesheets, etc.) in the page. After crawling the DOM, YSlow loops through Firebug's Net Panel components and adds those to the list of components already found in the DOM
In the second pass, YSlow gets information about each component: size, whether it was gzipped, Expires header, etc. YSlow gets this information from Firebug's Net Panel if it's available. If the component's information is not available from Net Panel (for example, the component was read from cache or it had a 304 response) YSlow makes an XMLHttpRequest to fetch the component and track its headers and other necessary information.
In the third pass, YSlow takes all this data about the page and generates a grade for each rule, which produces the overall grade.