I don't know what it is, but it seems like host is a disadvantage. How do you know if you have host? When the game starts, immediately hit the back button. If you're the only one without dog tags (while everyone has dog tags) you're the host. You'll have 4 green bars as well (usually pops up first). In other FPS games, when I would get host I would kill someone within 2 shots or so and the game would run like butter for me.
However, in MW3 I feel like I'm at a disadvantage. I'm not sure if it's the way the network is coded to make up for such an advantage. I played a TDM game (with host) and some people were skipping around as if I had a bad connection. I have Comcast @ 28MB/down and 8MB/up. It just seems I'm better off not having host because of how the game'**** detection and balancing works. Just wondering if people have had this happen to them with host. This game feels differently with host than other FPS games I've played.
PS - I get host about 40% of the time I play.
TL;DR I have host, but people still lag around me.