I mean seriously. Have you ever been on the EA FIFA forums? There's a gif as their avatar and about 20 in their signature. It doesn't lag at all but it looks bad and one of the reasons I like AG is because it looks so clean.
Kongregate allows gif avatars, not many people use them, but the ones I see aren't annoying. With how small armatars are displayed in the forum, I don't see it being distracting/annoying.