Even with volumetric lightning turned off, I get around 40-50 FPS with all lights turned on at the base (while being at the base). Turning everything off gives me 90+ FPS on average (while being at the base). Getting away from the base (even if the lights are on) gives me 100+ FPS on very high rendering/graphic settings (including volumetric lightning turned on).

The more lights I enable, the less fps I get (while I'm at the base). I don't need to perceive the light directly, walls and doors don't matter.

Surely there must be a way to optimize this?