My experience is similar so far. I suspect it's the radar code and Lantirn probably as well. FPS drops to 1-2 fps in the RIO pit with Lantirn active and when new terrain is ahead. Flying back, over already loaded terrain, it is a bit better. Need to test more, though, but double-checking with a quick jump into the FA-18 (same mission, same route, no lantirn though ofc) gave me the expected 30-40 fps.