Google, Amazon and Wikia have reported an increase in abandonment and decrease in conversions as page load time increases.
The average webpage takes 4.9 seconds to load. Most of the wait is on the frontend. The tools and best practices to reduce page load time are freely available. There is a strong business case to use them.
Fresh back from O'Reilly Velocity 2010, this is a presentation I gave to the Melbourne Ruby User Group.