You simply start by downloading the extension, then click the speedometer on your extensions bar, click run test, and it will run whatever website you are browsing on.
- It displays statistics about the page;
- It summarizes the page’s components;
- It offers suggestions for improving the page’s performance;
- It provides you tools for performance analysis, including Smush.it™ and JSLint.
- It grades webpage based on one of three predefined rule set or a user-defined ruleset;
It excels in giving you exact areas in which you can fix quickly. Works like Google Page Speed Insights but generates different set of rules based on the website you are browsing on.
How YSlow works?
It works in three phases to generate its results: First it crawls the DOM to find all the components (images, scripts, stylesheets, etc.) in the webpage. Once crawling the DOM, it loops through Firebug’s Net Panel components and adds those to the list of components already found in the DOM.
YSlow takes information about each component: size, gzipped, Expires header, etc. Then it gets this information from Firebug’s Net Panel if it is available. If the Net Panel information for component is not available, it makes an XMLHttpRequest to fetch the component and track its headers and other important information.
YSlow takes all data about the webpage and generates a grade for each rule to produce the overall grade.