SlateUI crippling my games' performance, only in VR.

I would love to get the thought of someone at Oculus about this one.

After testing some more it seems like this is tied to the mirroring window output.

I created a new Unreal default VR blueprint project. Didn't change anything except a toggle to turn on and off vr mirroring with a blueprint command 'vr.spectatorScreenMode 0'.
I then ran the same cooked build eight times, alternating the on/off state of the vr mirroring output window, and getting a unreal editor GPU profiling output dump to the log file using Shift+Ctrl+comma.
I restarted the game inbetween each take, ie 8 times.

This is what I got and you can clearly see that the mirrored runs consistently get an ~3ms penalty.

(the runs without mirroring don't get a 'slate ui' item in the profiler output)

This is the same result as I am seeing with my main game project.
It's not good as it means that I can't have the mirroring it's just too crazy expensive. I don't see why it even should be it should just be using already an rendered image and displaying that in a window.

Cheers
Fredrik