It goes into great detail, but the quotes at the top summarize it well;

“[Google found that] the page with 10 results took 0.4 seconds to generate. The page with 30 results took 0.9 seconds. Half a second delay caused a 20% drop in traffic. Half a second delay killed user satisfaction.”

 “In A/B tests, [Amazon] tried delaying the page in increments of 100 milliseconds and found that even very small delays would result in substantial and costly drops in revenue.”

 http://www.codinghorror.com/blog/2011/06/performance-is-a-feature.html