VR mirror screen low performance


I noticed there is a significant performance drop whenever I enable the default mirror screen mode in VR ( SingleEyeCroppedToFill)

Has anyone found a way to make it run faster? I can understand why spectator screens can affect performance (due to screen capture), but I don´t understand why an already rendered scene can tank peformance like that when the engine is simply displaying what already rendered for one eye. Am I missing something?