U4E uses ALOT of effects, like almost everything we could possibly think of. It's only to be expected that sometimes it will slow down in some areas. It's like, what do you expect?
On my system it's pretty smooth most of the time, and I only have an AMD 1.2Ghz processor. I've tackled DOZENS of monsters in Gauntlet at a great frame rate, and it still runs alright when Fortress is PACKED with 13 bots in Lava Giant.