You're doing it wrong. I can max out UT3 on my computer, any map, and still get more then 60FPS regardless of what's happening.
I'm not going to argue with you, but there's nothing to do "wrong". On the map where the green crap is in the middle, my FPS drops tremendously, also on all of our gaming computers at work. It's the game engine, aside from a couple areas like that, the game runs good.