I had this same issue. Tuned some of my settings a bit, and it seems my FPS drops mainly when photogrammetry is turned on. If I turn it off and leave everything else on, performance returns to normal. I discovered this when I went outside the plane and started panning around with my camera.
I also received a bandwidth limitation error while panning around. Which doesn’t make any sense because my bandwidth is unlimited. Maybe the photogrammetry data is somehow capped by mistake, but not by a setting in game, but rather in the flight sim engine code itself?
There’s something the main thread is doing which is causing the poor performance. If I turn off photogrammetry, the performance and main thread usage (or frametime) returns to normal.