One problem I have experienced (and experimented with) that is so bad, about half FPS, is when HPET is ON.
This problem seems to be more pronounced on Ryzen CPUs, and when I experimented with it (SU10Beta 1.27.16), there was a frame rate drop of about 60% with it ON.
There are many similar examples on IntelCPUs besides MSFS.
I have answered another person in the past and that person was not cured, but there are many cases that can be cured, so I will give you the link.
You can try it if you like.