Here are some useful articles that talks about how to improve the performance of ASP.NET Application. I also included here some links that talks about the best practices for speeding up your website.
Improving ASP.NET Performance
http://msdn.microsoft.com/en-us/library/ms998549.aspx
Best Practices for Speeding Up Your Web Site
http://developer.yahoo.com/performance/rules.html
10 ASP.NET Performance and Scalability Secrets
http://www.codeproject.com/KB/aspnet/10ASPNetPerformance.aspx
10 Tips for Writing High-Performance Web Applications
http://msdn.microsoft.com/en-us/magazine/cc163854.aspx
Good Luck!