I checked the "cloud issue" again. I still think the 2021 version is either more demanding in general and clouds make for a very significant factor, or clouds themselves take more resources. I tried with an F15 from Travis AirForce Base, cumulus cloud density and altitude max, flew to and through them. In 2020, performance dropped only a bit. in 2021 very significantly, most noticable when turning the plane or panning the view.
It's equally bad with max cloud densiy but lowest height, taking of with the EC135 from SFO hospital, in both versions.