Same problem...I had very poor performance under Windows 10, i5-9600k, 32GB DDR4-3200 CL18, RTX2080 with my VR HMD Pimax 5k+ using minimum quality level in DCS (set Min or VR profile) on a clear DCS instalation (without any mods), but with pixel density 1,5 for better panel reading (about 19-24 fps) and because of this reason, I haven't played DCS for a long time (about 6-7 months). Now I have improved my PC to i7-12700K, 32GB DDR4-4133 CL22, RTX 3080Ti and updated the system to Windows 11 (for Adler Lake features).
What I see now? Yes - I have good fps on the same minimum (!!!!) quality settings - or stable 45, or unfixed 65-70, but when I start flight and moving my head in VR, so image in VR HMD became is very shaky (stutters - moves by breaks). On the monitor the duplicate image looks good. When I press Alt+Tab in HMD I see "Windows out of focus", I can't to control the sim, but image looks great - the image moves smoothly, one might say, perfectly, there are no artifacts, but you can’t play, because there is no control due to the focus on the desktop of the system. As soon as you press the key combination Alt+tab again to control the DCS application, the image becomes jerky and it is simply impossible to play.
I see a lot of topics on this forum about this problem, but some peaple found a different solutions by third party soft. It's too bad that the developers don't pay attention to the fact that the game has a number of bugs in VR and they still can't fix it.
I have not yet checked the options for disabling motion smoothing in SteamVR, but in software from Pimax I turned off all the improvements and it did not bring any result. I want to draw special attention to the fact that, in my opinion, the point here is not a software conflict from Steam or Pimax / WMR, but the problem of the bugs in DCS game, since when I run any other game under VR, I have no problems with the image under Windows 11 - everywhere the image is rendered without jerks/stutters.