I have the same exact issue.
My Configuration is :
RTX 3090
5900X
64 GB RAM
750 W PSU
With the Fenix I start flights at above 30fps but when I’m landing the FPS is down to 22fps
For example : LIRF to EDLP both with the custom scenery mods.
This framerate drop also happens with the PMDG 738
Even the GPU is never pushed to 100% and CPU temperature maxes around 70 to 64°C
Please help me