I have experienced this problem before. It seems to happen the most when I play on servers that are running the Invade and Annex COOP mode. I usually play on these servers to practice with helicopters, so most of the time it happens I am flying around. It tends to happen after an hour or so of playing. I also play in a weekly private milsim PvP event that hosts up to 120 players for about 3-4hrs at a time and I have only seen the 3fps bug once in that setting. We never play with AI and we run a very strict and limited set of mods. I can't say whether or not I have played an equal amount of PvP to COOP, but I can say that I have experienced the bug many more times when playing with the AI. This is all anecdotal but I hope it is helpful.
EDIT: Ok after some quick math it would appear that I have played a roughly equal amount of COOP to PvP.
