Forza Demo Fullscreen tanks frame rate where windowed mode is great

Can someone help me. In windowed mode on windows 10 I can run the game on ultra no problems but if I put it in full screen it drops to single digit frames. I assume its a UWP problem.

I have i7 7700k 32gb ddr4 1080ti

Same problem with me. I just donwloaded the demo for PC.
When in fullscreen mode I have about 30 to 38 frames and the GPU is at 35 %. The game lags and stutters.

When in window mode I have about 60 FPS and GPU is at 70% and everything is smooth and good.

My System is:

Windows 10
i5 4570
16GB DDR3 RAM
1060 with 6GB

Same issue for me, but in the Ultimate Edition full release.

G-Sync monitor could be the cause maybe? Windowed mode runs perfect, but annoying.

21:9 @ 3440x1440p
1080ti