Is High Precision Event Timer (HPET) enabled? Is it disabled?
We have verified in the past that if HPET is enabled on Ryzen series, the FPS may be extremely worse on MSFS.
Disabling it improved the situation.
Method
Open a command prompt as administrator
bcdedit /enum
useplatformclock->Yes
Since it is enabled, disable it.
bcdedit /deletevalue useplatformclock
and reboot to disable
useplatformclock->No or there is no item
Disabled.