Best option to create real mirrors in terms of performance

I want to create several realtime and physically correct mirrors in a Unity scene. My main problem is the impact that they have on performance. The scene runs in a VR environment with a HTC Vive and once the mirrors are active, there is a noticeable worsening of performance.

The method I am using, is the shader variant "MirrorReflection4" (http://wiki.unity3d.com/index.php/MirrorReflection4) and so far the best I experienced but still very taxing, although I am using reduction of the resolution and frame rate. Besides this method, I also found these options for creating mirrors:

• Unity's ReflectionProbe (works, but even more taxing in terms of performance)

• Vive Stereo Rendering Toolkit (also big performance impact)

• Selfplaced Camera that is rendered on a plane by a Render Texture (not taxing at all, but not physically correct)

Does anyone know different methods I could try or has some ideas to reduce this performance issue?