Stereo off while prox sensor uncovered causes low framerate

Inputting 'Stereo Off' while the prox sensor is uncovered will disable stereo rendering but introduces large performance decrease. Stat FPS returns constant 10 FPS / 100 ms.

Found on:
//UE4/Release-4.15 CL-3334709
//UE4/Dev-VR CL-3334961

1. Run UE4Editor.exe QAGAME -game
2. Enter 'stat fps' command to display frames
3. Enter 'stereo on' command to initiate stereo rendering
4. Cover prox sensor on Oculus Rift (should be left uncovered until this point)
   1. Will begin rendering in stereoscopic
5. Open the command window and type in 'Stereo Off' Do not enter it yet
6. Uncover the prox sensor
   1. Falls back to rendering a black screen
7. While the prox sensor is still uncovered, enter the 'Stereo Off' command.

Result: Starts rendering in non-stereo again but is capped at 10 FPS / 100 ms.