As a part of the ASP.NET Developer Webcast Series I also talked about ASP.NET Performance and how you can write better web applications targetting performance when working with ASP.NET.
Here below are some very useful resources that help you with planning, measuring and tuning performance.
Improving ASP.NET Performance
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http://msdn2.microsoft.com/en-us/library/ms998549.aspx
10 tips for writing high performance web applications
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http://msdn.microsoft.com/msdnmag/issues/05/01/ASPNETPerformance/
Check List for ASP.NET Performance
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http://msdn2.microsoft.com/en-us/library/ms998596.aspx
In addition, I also wrote a couple of posts on ASP.NET Performance which you may want to check as well. Here below are the links
http://geekswithblogs.net/ranganh/archive/2006/08/04/87099.aspx
http://geekswithblogs.net/ranganh/archive/2005/04/25/37601.aspx
Cheers !!!