So it's not a simple equation of taking 2x the geometry and receiving 2x rendering time. Memory is even more critical - if the resources don't fit into your memory, there's not that much you can do.
Theoretically this maybe true, practical the user can remove the mirror glass of a little/medium bath room scene and mirror the scene as a raw workaround. Anything is rendered perfect and fast without any technical problem. Unfortunately this workaround doesn't work for any scene, because the mirror-room relations. Also my impression was that Enscape can render really complex and heavy scenes, but fails to render a mirror at a small simple bath room.