Amazon.com increased its revenue by 1% for every 100 milliseconds of load time improvement

"But for the precious milliseconds site owners can shave off page load times, they can see huge returns. For example, Amazon.com increased its revenue by 1% for every 100 milliseconds of load time improvement. And Aol said its users in the top 10% of site speed viewed around 50% more pages than visitors in the bottom 10%"

Source:  Stated in Mashable, 6th April 2011.  

Also quoted in this presentation from 2008

Note - Original source not found.  Another classic stat that crops up a lot.  If you know the original source please leave a comment!