Also have a 3070 here. Same issue. I always start WMR after launching the game and loading the flight. Then, everything is smooth until I exit and re-enter VR (or WMR crashes lol). Then, it becomes unplayable ! (also, it seems to be MSFS fault, because MSFS stays stuttering badly even if I quit WMR).
One could say “fine, just don’t exit VR”. Unfortunately, I usually don’t choose to : I have an AMD motherboard, wich means awful awful awful connectivity issues with the G2 (V2 cable but still). So I can fly smoothly until WMR crashes and then : game is over. No way to re enter VR and have a smooth experience.
This is extremely frustrating. I’m selling an organ and buying a 4090 next week, so I’ll come back to tell you if it fixes the issue (unlikely IMO).