I will follow also this as with the ORBX Germany (North, South it does not matter) I have at least 8-10 fps loss, and as you, when I deactivate them from the scenery (and the FTX Global and openLC Europe remain), frames are getting back to normal.
And to have more input for the developers, my system is:
i9 7900X (core 0 @4.6Ghz, other @4.4Ghz)
Asus X299 Prime Deluxe
EVGA 1080TI FTW3, latest driver
RAM DDR4 32 GB@3200MHz GSkill quad channel
P3D v4, Win 10 Pro
Samsung 850 Evo for the P3D and Samsung 840 Pro in Raid for Windows.
4K resolution, 30Hz set for the monitor (I always have smooth 30 fps but with ORBX Germany drops to 22 fps..average)
It does not matter if I test at an airport add-on or at a default airport, is the same fps loss.