YSlow grades web pages based on one of three predefined sets of rules (or a user-defined set of rules). It will offer some suggestions for how to improve your page's performance, summarize the page's components, displays statistics about the page, and provides tools for performance analysis, including Smush.it™ and JSLint.
Once you have installed the add-on into Firefox and re-started your browser, you will see the YSlow icon at the foot of your browser window. When you click on this you will see the following:
In addition, you will see that there are various scores for each area of interest and tips on how to improve the performance of your page. you will see that some areas were graded A, whilst some were as low as F. Your overall score takes each individual score into account and if you want to improve your page's score then the first place to look at is your lowest scoring ones, i.e. where you scored F (or E if no F's).
So, you can see that YSlow is a useful too to check problem areas with your site. It's also interesting to check other web pages and compare their performance to yours. You will be surprised how poorly some sites do. Even Yahoo's own page about YSlow only scored a B (and 87%) when I tested it!
