Were you over a super large airfield with a flak base near by?
I found in cloud front testing with DX9 that my FPS would drop from 60 to 47 while at a super large airfield with a heavy cloud front going over with tracers going off all over the place at the 6 drones. DX11 would make it a bit better. Then I removed the high stratus layer and my FPS was back to normal.
What terrain was this and what field or fields were you over?