I've found the answer. It appears the if you have IPv6 enabled on your PC and are using Firefox, viewing localhost sites runs dog slow (not just for Cassini - any web server)!
For anyone who runs into this, the answer is to disable IPv6 on your PC, or use
http://127.0.0.1 instead of http://localhost to refer to locally hosted websites. In IE this doesn't seem to be a problem.