mattur - 1:16 pm on Feb 15, 2010 (gmt 0)
I aim for < 300ms processing time on the server. Hardware, efficient database design, caching and code optimisation are all important.
Client side optimisation can also make a big difference for users. I've just done a new website for a client that is larger in download size than their old one (due to image-heavy rebranding) but faster to download for most visitors (big cut in number of HTTP requests).
YSlow and Page Speed firefox extensions are essential. Steve Souders' books and his blog are also highly recommended.
Make it as fast as possible. Then make it faster some more :-)