truth is a lot of 3rd party models have only the highest (closest to viewpoint) levels of details, and with complex, detailed models it's impacting frametares BIG time -example being Veltro's Stirling -put a flight of four or five and it becomes unplayable -I had to remove it from my WW2 install :(
The close formation flying and generally WW2 tight formations (bomber boxes) and airspeeds play key role in it too -planes close to each other, slow (compared to jet era) relative attack/pass speeds, makes the high detail, frame impacting models much more often drawn on screen, many of them -the only solution is to ask the modder who made the model to do much lower poly secondary LoDs (not so easy, and highly unpropable, often the creator's source max file is AWOL or modder won't have time will etc to do so)
with modern GPUs with say >512 megs or VRam the skins format isn't that inportant