I'm on dial-up so just about any of them except the most inanely simply bog me down. Don't make me go look for them - that's torture.
I will say, there are not many yet. The "batman" (not sure the user) one takes a good 30-45 seconds to load for me though. No offense meant to the owners of those avatars, and I don't want to get people pissed off at the forum, but I think a file size limit on avatars is a smart thing to impose for everyone.