Yes, I do understand all of that.
But I have tried making a mission with no AI, No planes in the air, no trees(yes to heard it right), changed affinity to 4, 3, 2 Cores!!, a flat dead landscape. No change in the framerate.
Removed most of the buildings(low settings) and when flying very low near any town, there is performance problems even when there are no buildings at all!!.
It's not just what you mention, there is something else in the code that is making the computer struggle almost all the time.