Cooperative combat lags

I can run this game on high visual settings no problems. FPS is 30 and more. In single-player - everything is fine and smooth. But in cooperative, with the same settings, everything is fine and smooth UNTIL combat begins. After that, the game starts to lag a lot, and fps drops to 1-2. After combat ends, everything is normal again. Tried lowering visual settings, installing new drivers - doesn't help for Cooperative. Whenever there is combat in coop, the game starts to lag.

And before you ask - NO, it's not a problem with my system. If I can run the game in any settings, including high visual settings, in singleplayer JUST FINE, but I can't do the same in cooperative, then there's obviously something wrong with the game. It doesn't even persist all the time, only when I'm in cooperative, and only when the combat begins. When there are no mobs - the game is smooth in coop, but when mobs appear in coop - the game lags.