Hey guys, I had exactly the same issue. Everything was fine during takeoff and climb, but once I reached cruise altitude, my FPS dropped to around 50. Both CPU and GPU usage were very low, even with TLOD set to 150. Interestingly, as soon as I started the descent, performance improved again, I gained about 30 FPS, even while using an airport addon in Antalya.
My flight was from Zurich to Antalya, using the iFly 737 MAX.
Specs: Ryzen 7 5800X, RTX 4070, 32GB 3200MHz RAM.