So I never lag in online multiplayer in this, or any other game, or any other game that has online co-op. Yet for some reason I get crazy lag when playing Halo games co-op online. Both Halo: Reach and Halo Anniversary were exactly the same. Anytime I'd try to play a game with my brother one of us would stutter and lag so bad it made the game unplayable.
Yesterday I tried to play Halo 4 with him and had the same result. My NAT is open and there should be no issue. I can play any other game without the slightest issue, it's always Halo that does it, and again only in co-op.