Assuming you are talking about Microsoft Flight Simulator, Windows 11 doesn't "mean" anything is going to happen to your FPS.
If you keep a solid backup image:
Let Win 11 install, test your fps, if favorable, keep it. If not, use your backup to revert back to Win 10. Eventually, though, you'll have to take Win 11 as long as your PC meets its criteria.
I read somewhere that it increased some FPS. IMO, it's too early to tell as these folks making these claims don't have the final release.