If it helps, I’ve started getting a similar issue as you @Marco2696
I can be in the sim for about 2 hours, including startup, FMC programming etc, then in the CRZ, everything is nice and smooth and then suddenly the FPS will tank right down to single digits. It can be the same place I have already flown over, which was smooth previously (same flight).
If I descend below FL290 it smooths out again, its bizarre.
HW is: Ryzen 9 5950X 64GB RAM NVMe Asus ROG Strix 3090 OC
The crazy thing is, when the FPS tanks, my GPU Clock Mhz drops by about half, during this time I can actually start up CyberPunk on max detail and it plays smooth, with MSFS tanking in the background.
Very odd.
EDIT: I am going to play around with live traffic / community plugins and see if its related in anyway. Not had this issue before.