Absolutely terrible!
I'm using Firefox at work on an HP 2510p Core 2 Duo ULV 1.6Ghz with 2gb RAM. No matter what I do, it gets unbearably slow after an hour or 2. Using Safari, IE6 (Yes, still IE 6 on XP) and Chrome poses no problems.
At home, it's an iMac 2.8Ghz Core 2 Duo with 4gb RAM...