I had this problem for a long time as well. Everything is buttery smooth until you actually engage with someone else, then it feels like all of a sudden the game is trying really hard to do something. It's usually a combination of pronounced network lag, then sputtering framerates. Last week I was playing and as soon as I snuck up behind someone within 10ft, as soon as I squeezed my gun, framerate literally dropped to 1-2fps which big freeze spikes in between, about 10 seconds later I was dead. I saw none of it.
It doesn't happen every time I fight someone but it tends to happen at least once every other game; maybe not always to that extreme but when it does it's enough to hinder gameplay.