Hey, what I would suggest doing is making everything you cannot see nodrawed. (Texture)
This is also a lot easier to do if you make your brushes with nodraw, that way you can easily texturize the places you can see, and keep the ones you don’t see nodrawed.
Also, make every complex, small or place you cannot reach func_detail. Honestly almost the only thing you don’t want func_detail is the ground and skybox + pluss walls that are sealing off rooms or similar
You can also lower the lightmap scale, this will give better FPS and all but it will make the light and shades look less nice. (something you probably don’t want for your map)
I hope this helps, though I am only speaking from experience.