Mark, thank you for stopping by and for your suggestion!

I have turned off everything I could find: I turned off all unneccessary render passes. I am just rendering 3D channels like distance, position and Index, Global Illumination, Glow, Background and Material Diffuse. I am not even rendering Bump, and just in case I turned Bump off in all the shaders. In the render options I am only rendering "Shadows". Nothing else! The Anti-Aliasing and lighting Quality are set at "Fast"Lighting Accuracy at 2px, so is Shadow Accuracy. Object Accuracy at 1px. Sky Light at 200% and Indirect Lighting at 100%, and I am still looking at over 30 hour render for my target resolution. Just for the room, no furniture, no characters. I just don't understand it.

I am not using textures whenever possible - nothing but black board and floor has actual image textures. Everything else is just procedural noises and turbulance mixers for colors or color gradients. The model is not super high poly either.

(The reason I want to render the room by itself is because I came up with this crazy idea of rendering the room as HDRI background using the spherical camera and then redering the characters with shadow catchers as floor and no room walls, but the HDRI instead of the modeled room for reflective light. But I can't even get a good render of the room within reasonable time....)