There's alot of slow-down during co-op play. There's so much coming at you that the Xbox doesn't seem to be able to keep up. However, you don't see the same slow-down in Deathmatch...of course you don't have all those enemies coming at you at once. The split screen is pretty standard, but I found it kinda hard on the eyes. You have so many enemies coming at you that it's hard (for me anyway) to focus your eyes on the smaller co-op screen to see what you're shooting at.
I know I'm in the minority on this game...it just didn't excite me like other shooters have.
