It might sound obvious, but a faster website is a better website — and Google and Amazon have gone to impressive lengths to prove this. A simple delay of 100 milliseconds was found to have an effect on Amazon's site and Google discovered the difference between a 0.4 second page load and a 0.9 second page load was a 20% drop in traffic.
Jeff Atwood over at Coding Horror writes:
I believe the converse of this is also true. That is, the faster your website is, the more people will use it.
A good read (as usual) for developers and website owners who want to know what it takes to make a website run as fast as possible.