http://developer.yahoo.com/yslow/

This is a FireBug plugin to find the performance of the web site.

Some points from the site about creating high performance web sites.

Web Performance Best Practices and Rules

Yahoo!'s Exceptional Performance team has identified 34 rules that affect web page performance. YSlow's web page analysis is based on the 23 of these 34 rules that are testable. Click each performance rule below to see the details.

1. Minimize HTTP Requests
2. Use a Content Delivery Network
3. Avoid empty src or href
4. Add an Expires or a Cache-Control Header
5. Gzip Components
6. Put StyleSheets at the Top
7. Put Scripts at the Bottom
8. Avoid CSS Expressions
9. Make JavaScript and CSS External
10. Reduce DNS Lookups
11. Minify JavaScript and CSS
12. Avoid Redirects

1. Remove Duplicate Scripts
2. Configure ETags
3. Make AJAX Cacheable
4. Use GET for AJAX Requests
5. Reduce the Number of DOM Elements
6. No 404s
7. Reduce Cookie Size
8. Use Cookie-Free Domains for Components
9. Avoid Filters
10. Do Not Scale Images in HTML
11. Make favicon.ico Small and Cacheable

» Check out the YSlow Ruleset Matrix to see how the grade is computed.