The performance of applications is a general concern in software development. In the particular area of web applications, the web pages loading speed is an important factor that directly influences the website success. From the theoretical point of view, there are many aspects determining the minimal time needed by web browsers to render a certain web page.

Many free tools to measure and optimize the web pages loading speed are available over the Internet. YSlow is an add on for Firefox web browser and works integrated in Firebug. It is provided by Yahoo! Inc. and it gives you a performance report by verifying if a certain web page code and components are in compliance with some essential performance criteria. By following the suggested performance rules, you can re-evaluate your website structure and repair the issues that decrease the web pages loading speed in web browsers.

YSlow verifies the next characteristics that determines a certain web page rendering speed: the number of HTTP requests, the presence or absence of a content delivery network, expire header, GZip components, the position of stylesheets and scripts relative to the top or bottom of the web page, CSS expressions, internal or external scripts and more. On the Yahoo! website, these factors influencing the optimal rendering speed of a web page are explained in details and, as a consequence, you will be able to identify the code structures or other page components that increase the time needed by a web browser to render a certain web page.

YSlow add-on will help you optimize your website in order to have faster loading web pages, but is also recommended to have in mind all the basic rules of web pages optimization (not only 14) and to use other speed optimization and testing tools as a comparison method in order to obtain the highest performance.