Regarding the aircraft hangars, everything is completely stock. I have never modded anything into the game, so it's safe to say the entire game is in its original state. Joining this forum is actually my very first time intending to mod SF2, and as of the time I'm writing this comment, my SF2 is still basically 100% stock.
I experience this on almost all terrains like Vietnam, Europe, Israel, Desert... Just panning the camera towards the hangar during takeoff drops the framerate, but if I look in another direction, everything is fine especially when I zoom in close on the aircraft with the hangar in the background. And as I mentioned above, the framerate takes a massive hit, dropping below 20 FPS, when the area I'm flying in has bombers or attackers performing ground strike missions in a custom campaign. This happens even though I don't include more than 40 aircraft total (counting both Red and Blue forces) in the battle (it affects both sides). Furthermore, if I fly near positions with multiple KPV/ZPUs or ZSUs firing simultaneously, the only way to describe it is an absolute disaster for the framerate. Yet, other AA weapons like the KS-19, no matter how numerous, do not cause this issue at all. It's incredibly strange because this FPS drop seems to happen selectively.
As for my specs, I have 6GB of VRAM and 16GB of DDR4 RAM. Regarding VRAM, I'm not sure about the remaining capacity while the game is running, but monitoring software shows the game doesn't actually consume much system RAM. In fact, it runs lighter than when I'm using Google Chrome or opening Discord.