The problem is that the encoding of the renderer and the internal render target of Reflector are different. That means all shader programs are recompiled per frame which of course totally kills the performance.
If you need sRGBEncoding, you can fix the issue by adding encoding: sRGBEncoding to the following parameters object in Reflector:
Please verify first if the above approach fixes your issue. If so, I think it could still be valuable to report this at github. Ideally objects like Reflector could automatically derive the correct encoding from WebGLRenderer in some way.