Having the same issue.
The FPS at my arrival airport is about 40-50% lower than when I take off.
Seems to me that this might be an issue where the memory isn’t cleared of old scenery correctly and starts overflowing at some point.
I do have decent system and graphics memory though:
7800X3D / 4090 / 64GB 6000mhz
Hopefully Asobo is aware of the issue and will look into it / fix it.