Speeding up an ASP.Net Web Site or Application
- Script Combining in .net 3.5 SP1
- Best Practices for fast websites
- HTTP Compression (gzip)
- Compress JS / CSS (different than http compression, minify javascript)
My best advice is to check out the YUI content. They have some great articles that talk about things like CSS sprites and have some nice javascript libraries to help reduce the number of requests the browser is making.
If you are using Firefox to test your website, you might want to try a nifty Firefox extension from Yahoo! called YSlow.
It analyzes your web pages and provides grades from A-F (A being the Best and F being the worst) for each of the best practices, for high performance websites. It will help you to track down the elements of your website which you could optimize to gain speedups.
UPDATE
Now YSlow extension is compatible with all modern browsers such as Firefox, Chrome, Opera, Safari and others, read more here.
Turn viewstate off by default, it will be a night and day difference on even the most simple pages.