That is limited by GPU. I believe with VR you are essentially rendering twice, one for each eye. Its almost like having a screen capture 2d in there rendering every single frame. I believe the Oculus Rift requires somewhere around a GTX 770 or R9 280x in order to run sufficiently. I’ve got two 780Ti’s and using just on screen capture 2d limits one( I only use 1 in engine) to around that 75fps mark. Best I can suggest is to get more frames lower your shadow quality and screen percentage.
Rendering in game with VR or a scene capture 2d is exactly a direct trade off either. You can have a render target at 19201080p and reduce it tenfold down to 192108, and you’re not going to get many frames back, its not the same as recording footage with lets say MSI afterburner. The engine is rendering every single thing over again, effectively it will cut your FPS directly in half if it has to capture every frame(which with VR you do), resolution will have little affect, but reducing shadow casting objects, or the shadow quality in the baseScalability.ini file and fine tuning it quite a bit, can get you a lot closer to that 75fps. I haven’t used VR much because of how expensive it is for GPU’s but it is pretty high up there.