The mirrors create absolutely massive performance issues, and should have a config option to disable the visual effects.
When not facing them, the framerate is in VSYNC. When facing them, the framerate drops to ~20 FPS. And this is with OptiFine.

Previous release should address that, mirrors no longer use TESR rendering and all the light beams are now part of the boiler's renderer which on its own should be a massive improvement. If that's not enough then I'll probably add a config to turn off the light beams completely later on.