Same problem here aswell. It slows down/feels laggy like the Docks area of the singleplayer maps. Netgraph shows frames from 60 to 15, which isn't really good for that engine and the small Arena maps.
My system is still Intel D830 (3ghz dual-core), 2GB Ram, GeForce 6800gt, Soundblaster Audigy 4
All drivers up-to-date and no beta drivers installed, like always