Webpage loading time is one of the most important parts of a website that directly influence its success. If your web page doesn’t load with in a specific time, precisely saying with in 8 seconds, users are likely to lose interest and navigate away.Web page rendering time is different in different browsers and there are many aspects determining the minimal time needed by web browsers to render a certain web page
To measure and optimize your web pages loading speed, there is an add-on for Firefox from Yahoo! – YSlow. It analyzes web pages and tells you why they're slow based on the rules for high performance web sites. It works integrated with Firebug a popular web development tool for Firefox.
YSlow's web page analysis is based on 13 identified basic rules that affect web page performance like the number of HTTP requests, list of components in the page, expire header, GZip components, the position of style sheets and scripts relative to the top or bottom of the web page, CSS expressions, internal or external scripts and more. Full details
YSlow helps you to optimize your website in order to have faster loading web pages by 25-50% with 13 rules described.[source]