I think that when you get AI going into a furball, with the computer juggling all the planes flight paths and actions (firing, creating graphics, damages computations, rendering scenery, clouds, etc), that the AI will effect it a bit more.
I am addicted to a new space sim called X3 Terran Conflict. Its much like FS and CFS but in a space environment, and interactive, not just free flights, so you can haul stuff from whomever, create bases, do nothing but dogfight the badguys, etc.. When you get into furballs in that sim, you have to lower your settings a bit or your computer slows down. This could be a temp issue on my own rig, but it happens. I lowered my primary graphics shader on it to Medium notch and its fixed.
It seems like the LOD of the objects in the environment in X3TC lower also when you lower the 'Shader' setting, which seems to lower polygon structures (auto-simplify on the object mesh?). With something like that, you could run such a game with a lowered poly structure setting on a really slow rig and probably run it smoothly. Thus, you could run it smoothly on a laptop. Its an amazing feature. The full out max setting has full bump mapping, but with medium setting, its flat textures, which means somehow, they must switch off the multiple material textures to one or two layers instead of 5 or 7, (specular, occular, bump, diffuse, ambient, light, etc).
Imagine a version of FSX, where you could max all the settings but have a single 'mesh/shader' setting that is 'simplified' and would allow you to run FSX full out on a laptop on battery mode..
I love talking about this stuff.. Gets my blood going... What if.........?