First and most important: Star Citizen is not officially supported, like the start message indicates the provided profile is solely for users who like to experiment with the game nonetheless. The game is still in alpha, vorpX might stop working with it (again) at any time.
To answer your question: rendering games in stereo 3D costs about half the performance you normally would see on a monitor since everything has to be rendered twice, in addition to that there is some extra overhead for sending the rendered image to the headset.
Performance in Star Citizen should be good enough for playing Arena Commander missions, but in the persistant universe you probably will have to switch to Z-Buffer 3D in the vorpX menu, which doesn’t look as good but is a lot faster.