Ok, so the testing continues. Keep the AI off for the moment and reduce the overall texture size of FSX by going to options, graphics and setting global texture resolution to one or two notches lower.
This will really harm your optics, but since we're just testing stuff, it may be ok.

Regarding the traffic, please check again if the 0% slider setting actually did set all AI to off. You can see that in the Views > new view menu where no other aircraft should be available then. So the menu for air traffic shouldn't be there since there is no other traffic.