This is a bad bug. Going to wait till it's fixed, hopefully soon.
Couple of things I've noticed that might help others
- In VR the frame rate is locked to the refresh rate of the desktop monitor. If you ALT-TAB in the desktop once in the VR view in the headset then you'll see better frames. I have a 4K 60Hz limited monitor on my desktop, and the WMR HP Reverb I use is then locked in-game to 60 fps in VR. But if I minimize the game desktop window out of focus then it jumps up to around 80 fps (i9/2080, was hoping for a solid 90 fps even on low ☹️). There is no sound when out of focus (there is a setting for that in Settings/Audio in-game) but it also causes some other issues, like the throttle indicator value in-cockpit doesn't update to reflect throttle position etc., so basically you'll trading off other display bugs for this.
- In SteamVR, at least for Windows Steam VR, you have to set the Motion Smoothing to 'Forced Always' to kick in the equivalent of ASW. Auto or On doesn't work. The latest beta of WMR for SteamVR lets the SteamVR settings be used as the underlying settings UI (rather than the default.vrsettings file), so you can at least do it 'per app' on that side. That will lock it to a solid 45 fps and reproject as 90 in headset. I guess that's the best workaround for now.