The freezing has nothing to do with the UGC, it's just a buggy game. It seems like the freezing depends on the server. Like right now I just tried to join a vCTF match on a Necropois server with 14 people in it, but it freezes each time I try and join. I guess you just gotta rememer which servers make your system freeze and try not to join them. Hopefully they add a "Favorites" section in the server browser.