DX:IW was designed for the XBox.
Newsflash: It runs even worse on the Xbox than it does on the PC. If DX:IW were designed specifically for the Xbox, it would be a lot smoother. The problem is that it was designed to hit a middle target. It should've been made for one platform and then ported to the other. That way, each platform gets its needs addressed. Creating a single game that works on two different platforms just results in too many compromises. EVERYBODY loses with multiplatform games.