I'm not a network gaming expert, but I'm assuming you're only as fast as the slowest part. Meaning, if the p4 slows down, so does the C2D. This may not be the case, but I've experienced it in the past with one of my slower computers on different RTS games (Dawn of War to be specific). The game would start to slow down (no lag or connection issues) just because of the amount of units that were one the playing field and in turn the game clock would slow down. This was on internet games and not LAN, but I think the same thing may be happening here.