Today I read a great presentation from Joshua Bixby (Strangeloop). He showed a nice way to create a business case for Web Performance Optimization (#wpo). The case is that a slower user experience leads to less revenue, but it's not easy to prove that. What you could do is set up an A/B test with a slower page as B variant. The upside is that you can make a business case, the downside is that you will lose revenue. Joshua came up with another option: we already have the connection speed and browser version in Google Analytics!
The older browsers and slower connection types have a negative impact on page load times. So they should show us numbers for people with a slower user experience.