If you haven't already done so, I would, while in VR - go to graphics settings and turn everything down to the bare minimum and see if that affects your FPS. The VR version is a separate executable, so the graphics settings in the 2D version are separate and different form those in VR. Just seems like there could be some setting that is set too high. I don't use shadows in VR, they are too much of an FPS hit, and my Environment slider is set to '1'.
Also, I'm on my 2nd Reverb cable and it gives me and error 1-4 nearly every time I fire it up. I have to disconnect the power block and re-plug it back in. I thought it might be a power saving mode incompatibility because I use "SLEEP" mode quite often, since my screen saver won't function when I have all my peripherals plugged in.
Sometimes even that won't help, and I'm forced to try every trick I know - rebooting or shutting down, disconnecting the headset completely and reconnecting using different display port, etc. I can usually finally get it to work, but not always.