I have the exact same issue with an RTX 3080 10GB.
Alongside the exact same discoveries as the OP, this is what I see when I enable "Display FPS" in Developer Mode:
It's not that I don't expect low FPS in KJFK, it's just the long FPS drops that shouldn't exist, unless there is something large loading in which visibly isn't the case. My settings are perfectly fine for a 3080, in fact I could go a lot higher. Even using "Medium" graphics settings introduces the problem. Default KJFK scenery is perfectly fine. Disabling nearly all scenery components doesn't change anything (obviously that would help the CPU, not the GPU) so it has to be something to with the textures. Please release a version with 4k textures, as 8k is completely unnecessary and as evident terrible for performance in this region.
FYI my CPU is a Ryzen 7 5700X.
