OK, my sincere apologies. The fault was on my side.
The said notebook was in power saving mode since my last train ride and had not noticed this.
So I re-checked now and the described issue is neglegible now. I have 45fps with WMR off and 35fps with WMR on. So I simply don’t notice the difference if I have the fame rate capped to 30 fps for Vsync as I usually do.
Also, in the notebooks “normal” mode the desktop window manager only takes up like 7% GPU resources according to the Windows task manager when WMR is on, leaving enough for FS2024.
Longs story short: I declare the case closed.