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Response times, availability, and stability are vital factors to bear in mind when creating and maintaining a web application. If you’re concerned about your web pages’ speed or want to make sure you’re in tip-top shape before starting or launching a project, here’s a few useful, free tools to help you create and sustain high-performance web applications.

I’ve tried to include a wide variety of tools that are easy to use, and have tried to keep them as OS and technology-independent as possible so that everyone can find a tool or two.

1. YSlow for Firebug

YSlow grades a website’s performance based on the best practices for high performance web sites on the Yahoo! Developer Network. Each rule is given a letter grade (A through F) stating how you rank on certain aspects of front-end performance. It’s a simple tool for finding things you can work on such as reducing the number of HTTP request a web page makes, and compressing external JavaScript and CSS files. A worthwhile read is the Ajax performance analysis post on IBM developerWorks that outlines practical ways of using YSlow in your web applications.

2. Firebug

Firebug is an essential browser-based web development tool for debugging, testing, and analyzing web pages. It has a powerful set of utilities to help you understand and dissect what’s going on. One of the many notable features is the Net (network”) tab where you can inspect HTML, CSS, XHR, JS components.

3. Fiddler 2

Fiddler 2 is a browser-based HTTP debugging tool that helps you analyze incoming and outgoing traffic. It’s highly customizable and has countless of reporting and debugging features. Be sure to read the “Fiddler PowerToy - Part 2: HTTP Performance” guide on the MSDN which discusses functional uses of Fiddler including how to improve “first-visit” performance (i.e. unprimed cache), analyzing HTTP response headers, creating custom flags for potential performance problems and more.

4. Cuzillion

Cuzillion is a cool tool to help you see how page components interact with each other. The goal here is to help you quickly rapidly check, test, and modify web pages before you finalize the structure. It can give you clues on potential trouble-spots or points of improvements. Cuzillion was created by Steve Saunders, the ex-Chief Performance at Yahoo!, a leading engineer for t