I have the same issue. I have GeForce 1080, Core I7, I play GTA with all ultra graphics and i have 70-100 fps. While of duty I have the same 70-100 fps, bur when I go on duty with plugins: ultimate backup, stop the ped, computer +, Deadly weapons, traffic police and with car pack I have 20-34 fps in-game (( In addition I have graphics mod Redux.