Significant FPS Drop when ingame Windows moved to 2. monitor

DX12 just decrease the CPU load compared to DX11, but its the same.
It need to check windowed mode. My actuall testing path was with fullscreen only.
I guess the ISLC Tool is helping a little bit, but only for the long stutter. But i have to confirm this as well.
I made a 4hrs flight yesterday and tried to find out what exactly could be the problem but so far nothing really found.
I made a video from the frame drop during moving OBS from one monitor to the other. It’s similiar to other movements and toogling. You can clearly see, that it is not a “limited by main thread” only thing and my GPU and CPU are sleeping meanwhile.