graphics wise, the main performance killer is the vehicles themselves. you can test this hypothesis by creating a level with you and a few vehicles parked around you, then remove them and compare the performance difference. my fps dropped 20 points with tanks and apcs parked around me. -40 or so fps with f15es and a10s around me. normal operation with just buildings and environment my fps is around 80 without vsync. but i play with vsync on so i don't feel the impact as much.
at 200k poly per dcs standard vehicle, a scene with five 200k vehicles parked together so that lod0 is on, generates a million polygons.
it's an all too common an occurrence as well in this game when flying formation. however, to their credit, they have a robust lod switching system so it's not really 1mil all the time, but more like 400 to 600k polygons or so when flying formation, and a constant 800k in airbases with vehicles crammed together, but not counting the environment and buildings.
most games like battlefield/cod, and something closer to dcs spectrum, like arma 2, will use vehicles at around 10k-50k(arma 2 aircraft carrier) polygons at lod0, but they use normal maps to offset the quality difference, which if using the high poly baking method, can yield even better results than a plain 200k model. that is easily possible at dcs level for something like a 50k vehicle using normal maps baked from a 1mil poly sculpt.
i'd say dcs is running double to triple the amount of polygons than the average game. the performance achieved is commendable actually, but i think it's a shame that they couldn't optimize the models more and free up resources for other calculations. and i've noticed that my video card is way louder and hotter when running this game. so the excess electricity usage from all the extra work load could have been avoided as well.