Getting the same issue here, just installed and sat down to play, going from 60fps to 30-32fps (seems lower than that though). Also only happening inside the cockpit and not externally.
Specs:
Gtx 970 i5-6500 3.20 ghz 8GB RAM
Settings are as follows:
https://imgur.com/a/OoF66
EDIT:
The apparent workaround with no other aircraft in your vicinity doesnt work for me.
Also worth mentioning that I get 60+ FPS with the A-10C and all other modules but getting 20-32FPS with the harrier with random spikes to 60FPS.
EDIT 2: Just switch airports from kobuletti to Vaziani and my FPS has shot up back to the normal 60FPS...Strange, I saw a post on Reddit about it being a bug at certain airfields, I guess thats it?