@ Sgt_Nick_Fury : The reason for the slowdown is that for Stereo 3D rendering everything has to be rendered twice, so a ~50% performance hit, usually a bit more due to some extra overhead for rendering to the headset, is quite normal for Geometry 3D.
You can trade 3D fidelity for speed by switching to Z3D in the vorpX config app. Doesn’t look as good/natural, but is a lot faster since it does not have to render everything twice.
In regard to Enderal specifically your best bet is probably to reduce draw distances. Try the “medium” quality preset in the Skyrim launcher. I don’t know Enderal, but if it’s anything like Nehrim, the prior Oblivion mod from the same team, they may show a lot more stuff per frame than the base game, which has a severe impact on performance.
The mirror display does not have any adverse effect. If Steam complains about that, you can safely ignore it.