Hi,
Same issue while having oculus mirror + obs!
I figured out that all of my VRAM was used! (11GB rtx2080ti)
The only way I found for recording is:
Link
OBS capturing game window (the screen with the 2 eyes)
Croping the window in obs only on a rectangle on the right eye
I also tried steam vr + capturing openvr in obs same issue: VRAM 100% = stutter/lag