Yup, there is a reason.
It's all to do with friends
Basically, to my knowledge, BO is constantly updating.
Every time you get a kill, complete a challenge or a game, it updates.
And so, with each of these updates, it's also sending out the info to all your friends, and vice versa.
As a result, your almost always recieving updates, this puts massive strain on your connection, resulting in it being lagging, regardless of your connection.
As of now, the only way to fix this is to either delete friends, or start a new account.
I'm not sure if they're working on a more permanent patch or what, but that will work for the moment.