Recently, I have been exploring the Microsoft Patterns and Practices series.
Improving .NET Application Performance and Scalability
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnpag/html/scalenet.asp
This is an excellent article that exhaustively examines the need for performance and scalability in .NET applications.
It is kind of the scientific method of .NET application performance and scalability.