It really depends upon your settings. If you press "interact" you'll see the FlyInside Tag appear which contains two FPS values like "FPS: 40/90". The second value is async timewarp, you want this to stay at 90 or very close. The first value is simulator frame-rate, which is really up to you. Higher frame-rates feel smoother, but you can also turn up graphics and fly with lower ones. I would recommend that you adjust graphics so that the first number is 50+, and the second number ~90
To increase frame-rate:
1. Make sure you are opted into SteamVR Beta.
2. Make sure you have the very latest NVIDIA drivers. There was a driver performance bug that was patched recently
3. Turn off vehicle and air traffic entirely. These can be a big performance hit
4. Turn down scenery complexity
Once you have a frame-rate you are happy with, start turning up the settings you care about most, to achieve a balance you are happy with. Then, experiment with increasing clarity by turning up resolution and MSAA from FlyInside Settings.
Hope that helps
It really is about playing around and finding the balance you enjoy most between graphics level and FPS