CustomerCentrix

There are several good posts about why people should tune their apps. Lots of other good places to find info on HOW to tune.

Web performance tuning does a great job of using case studies and research by the big companies to prove the correlation between web performance and profit. It's obvious - slow pages make users go away; thus, revenue is lost.

Top Ten Web Performance Tuning Tips
by Patrick Killelea, author of Web Performance Tuning is a good read for ideas. Patrick rightly points out that too many times performance testing is overlooked.