C6900 Lag

I recently purchased a new C6900 cable modem/wireless router from Costco to replace an aging Motorola modem/N3000 wireless router combo that was beginning to fail (reboots, crashes, etc)..

At any rate, the C6900 is "working" and testing the internet speed shows my full bandwidth I'm paying for at Cox. However, my son is complaining mightily that his games are now seriously "laggy". Outwatch (I think) or Overwatch... Not sure.  Anyway, he says by the time he moves, he's already dead.

 

Thoughts on what the issue might be?

 

TIA,

Owen