I didn't know that scenery was still calculated even when not
visible. I will dial everything down.


Not all of it is, but some. E.g., the dynamic stuff is still calculated
in the background in order to account for when a dynamic item will get
close enough to be shown. Also, the cloud and scenery detail settings
have an impact on how detailed the "fog" is shown, how detailed
lighting effects and shadowing is. These all use a lot of CPU. And
finally, even if you only see fog, the program might still "think"
there is something vaguely visible through the muck - and that needs
calculating, too.

Oh, in case you're impressed by our resident non-expert - the above
information is straight from members of the MSFS programming group,
which I had the pleasure to talk to.

--
Thomas Borchert (EDDH)