when i upgraded my PC all the other systems in the house slowed down, it wasnt' that i was using bandwidth, but the new system's transfer rate made it harder for the older systems to get in to "queue" for some reason, I forget how i resolved it though...... it was by either upgrading the older comp(>10yrs old), or by capping the wifi(to my PC), or a Net card update
I wish i could remember, but it was few yrs ago.....
My xbox now doesn't lag anything ever............on my network with another xbox and 2 pc's.