Issue: Lag during Spartan Ops and co-op campaign.
Halo 4 uses 2 different network models for online play. Spartan Ops and Co-op run on a “synchronous” network model. Let’s say you are playing 4-player co-op. One player is the “host”, the other 3 are “clients”. All 4 players load the level, and all 4 players start the mission at the same time. Now, when a “client” presses a button, such as ‘jump’, their Xbox sends a signal to the “host” Xbox, telling it what the client player is doing. The host Xbox receives these signals from all 3 clients, then calculates every movement, bullet fired, damage done, etc, and sends the updated information out to all the clients, who’s Xboxs then reproduce the changes. But, the host will only send out these updates when it has received signals from all 3 clients. It does this to maintain accurate and consistent information across all 4 players (In other words, all 4 players are seeing the same things in the same places at the same time).
This is where the lag comes in: If just one of the players in the game has a bad connection, it slows down their ability to send information to the host, which means the host has to wait longer before it can send out gameplay updates to the other players. So, even if you are a client with a great connection, and the host has a great connection, you can still get major lag if one of the other clients has a slow connection.
This is experienced as a sort of input delay (the game will not instantly react to your button presses).
Check the link for more details on War Games an multiplayer modes.