OpenGL deprecated in nDisplay

When I use nDisplay on UE4.23, it tells me that OpenGL is deprecated and I should use another render target. Since when is that and why? OpenGL is necessary for me to achieve higher frame rates as well as the only configuration that seems to work with stereoscopic top-down view.
Now my frame rates are terrible and my GPUs are only working at 30% instead of 100% with stable frame rates on UE4.22.