Choppy graphics in windowed mode compared to full screen although displayed FPS is equal

Hi,

maybe someone experienced that problem before: I have been using windowed mode for many years now in MSFS with no problem. But today I noticed it felt much more laggy. I was looking at the FPS but got around 50 as usual and also no unusual red or blue spikes. After some experimenting I noticed that in full screen mode I get the usual smooth experience. Anyone knows what could have happened here?

MSFS2020/AMD 7900X/GTX 3090Ti/32 GB RAM/DX12/TAA (but also happens with DLSS/DLAA)