In my case, when turning FrameGen off in DX12, I get 25% more FPS (counted without doubling for FG). When turning FG on again in the menu, my standard MSFS FPS decreases about 25%. I used the MSFS Dev Mode with FPS counter on to check this.
So at at RW 25C EDDF, FG on I get 80FPS (without the FG count), with FG off I get directly 100FPS sitting at the RW25C. While flying around also.
Can somebody please test this also?