I don't know why or how this works, but I have done it a dozen or more times and it does.
If you load MSFS 2020 normaly, and then close it 'back to your desktop' without running anything, reload it straight away and I found a significant rise in frame rates.
I had a good frame rate anyway, and I found this by accident.
Perhaps its just my machine but it would be interesting to know if it works on other computers.
PS: Has anyone an explanation for this ???