noname128 Hmm, the monitor settings like gsync/vsync would not effect the headset, and I have SteamVR set to the same options with automatic "smoothing" and 100% super sampling, so not sure it would be that either.
I'd listen to Philip before you take a look at any of the below, but a few more suggestions.
You mentioned that DirectShow seems "forced on". For me, it's just an on/off toggle like all of the other settings in DeoVR, sort of greyed out when off and lit up green (or maybe lit up blue? going from memory here) when on. Are you able to turn it off? Because just like DirectShow can help certain videos play more smoothly, it can sometimes have the opposite effect on other videos.
Have you tried playing videos in any other apps? Maybe try downloading SLR's app (which is virtually identical to DeoVR), or better yet another free VR player like Play'a or something similar. That might at least help narrow it down to potentially being a Steam or settings issue vs a GPU or hardware decoding issue.