Sure, there is. A Faltgame is rendered onetime. Only 1 picture is shown.
In VR there are 2 pictures to watch a game in Stereo3D. 1 picture for each eye.
So in VR it needs to be rendered twice.
More work the the GPU and more needed space of Vram.
Most VR-Games are not looking as nice as Flatgames because the performance impact.

For VR-Games one needs a good PC. For Flatgames mostly a Midrange-PC is enough.

Originally posted by 2⊛2:

Sure, there is. A Faltgame is rendered onetime. Only 1 picture is shown.
In VR there are 2 pictures to watch a game in Stereo3D. 1 picture for each eye.
So in VR it needs to be rendered twice.
More work the the GPU and more needed space of Vram.
Most VR-Games are not looking as nice as Flatgames because the performance impact.

For VR-Games one needs a good PC. For Flatgames mostly a Midrange-PC is enough.

so when i play a flatgame on steamVR there is a noticeable perfomance drop? (using steamVR as a virtual monitor)

Guess you mean you want to play a flatgame in SteamVR Theatre-Mode.
Expect a performance-drop but not as much as a real VR-Games in VR-Mode.
Because a flatgame will still a flatgame. You won´t see the flatgame in Stereo3D.
Best way is to try it. Then you know if your PC is able to manage a flatgame in VR.
Good Luck :)