YSlow analyzes web pages and tells you why they're slow based on the rules for high performance web sites. YSlow is a Firefox add-on integrated with the popular Firebug web development tool.
Many web pages are now moving to a "Web 2.0" style, where the components in the page and the page itself are built dynamically in the browser using JavaScript. YSlow analyzes the page's components, including downloaded components, using advanced techniques. For example, the front page of Yahoo! News might display an image on that dynamically rotates through one of four "top photos of the day" by setting the image's src to the different image URLs. YSlow measures performance for sites that use these advanced techniques and reports when the dynamically loaded components are missing an Expires header.
YSlow analyzes any web page and generates a grade for each rule and an overall grade. If a page can be improved, YSlow lists the specific changes to be made.
YSlow calculates the total size of the web page for both empty cache and primed cache scenarios, as well as information about cookies.
YSlow lists all the components in the page including their type, URL, Expires date, gzip status, load time, size, and ETag. You can also view the HTTP response headers for any component.