in MP (drop1 on screenshots):
1) i played insurgency (but im sure mission doesnt care + i was on base, so only some friendly AIs loaded)
2) move inside white building with ammoboxes
3) and go back outside - FPS drops to half
4) open main menu and see a miracle! fps is back in normal
5) close main menu and cry again :(
+ drop2 is same mission, just took heli.
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in SP:
1) just fly jet
2) when game starts loading new parts of map, open main menu - it should give you more FPS