The same happens to me, and I noticed something else: during night missions it gets even worse. Sith argument that VRAM could be the cause does not apply to me with my current rig:
iMac 27" running Bootcamp with Windows 10 64bit Pro, GeForce GTX780M 4GB , Intel Core i7-4771 @ 3.50Ghz, 32Gb DDR3 on a Fusion Drive 3TB.
As most of you guys said before, everything goes just fine until the TGP's FLIR is switched on. I should mention as well that frame rates are better in the DCS 2 Open Alpha, it still has the issue of FPS dropping 50% when the FLIR is used. Said that, DCS 2 gives me way 70% more fps with better settings than 1.5... maybe it could be a programming bug?
If you need better info, let me know.
Regards,