Thank you for your interest and answer. But as I wrote, this issue does NOT appear in airports from other developers with the same aircraft in use.
As explained, so many times already, it's just wrong to compare different airports located in difference places. In addition to that, in this case it's even more wrong, since there's nothing to compare against with, namely a fully PBR scenery not derived from an old product, since there are none out there, and surely not of this size.
As discussed several time, and of course as explained in the KORD manual, a proper PBR scenery ( or aircraft ) will require more VRAM than a non-PBR scenery, this because each object will require the full complement of Albedo+Metallic+Normal textures, while non-PBR products often use just a Diffuse texture.
The "other airports from other developers" might just be taking a bit less of VRAM, and with your settings and the rest of the loaded stuff might just barely fit your available VRAM
This means, if your VRAM has already partly taken by other stuff ( the airplane, the surrounding scenery ) combined of course with your settings, which might have been set too high because you tuned them for non-PBR products, if you cross the boundary between "everything fits in VRAM" and "not everything fits in VRAM", the fps will go down a lot.
As for vram, I simply can't imagine FSDT as intelligent professionals starting up a whole new project dedicated entirely for P3Dv4 would act as complete maniacs and build in a waste of unimportant details being too hard to handle even for this new powerful PC of mine....at least I don't hope so!
That's why we put the minimum requirements to use the scenery in ifs full resolution at 8GB of VRAM, and why there's a section on the KORD manual which explains which settings might affect VRAM consumption the most.