At P3D options I had ticked the box for "use high resolution textures". Unticking the box solved the issue with the framedrop.
Interesting, I always had the option enabled.
Do you also have a very high "Level of Detail Radius" for Terrain ? The two are likely combined, but I never put it on the maximum value, because it can have a very large impact on VRAM consumption. I always keep it at "High".