After I installed Panther, Safari and Mail would not load a page or email. It would try to load a page then stall. Being a fan of fonts I had hundreds if fonts loaded. Removing most of the fonts corrected the problem.
[robg adds: Can anyone else confirm a slowdown due to excess fonts? I have about 100 installed, and don't see any slowdowns here...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031026140729513