I had a similar issue personally, but it stopped when I ran the faf client as an admin + 4 core affinity.

This didn't resolve the issue for my friend though. He says that there is no lag when running the campaign directly in steam, IE not with FAF.

I can confirm that I have also had the same issue when playing with a friend. The Issue seems connected to the in-game "taunts" and other voiced videos. It seems to get worse if the camera also needs to "pan". And sometimes results in "crashes"/"freezes".