what bogs down?   there are only occasional reports on this and its from very low end servers…  obviously the forum requires more resources than just wp since its on top of wp…  what kind of server do you have?  dedicated or shared?

a couple of thoughts on performance…

dont show too many forums on the group view page…  dont show too many topics on forum view page…  dont show too many posts on the topic view page…

are you showing gravatars?  if show, you should be using our gravatar caching plugin…

have enabled the combined css and js caches within forum – options – global settings?