Dark scenes take also a lot longer to render. If you want to make a scene faster to render then add more light, so that the surfaces are hit by light more often and they resolve (converge) more quickly. You can alway adjust the aperture to turn the image darker. If light doesn't hit a certain point of the image then it takes longer to render it.

This is the same that is done in photography/cinematography. If you shoot only with practical lights you often end up with an unusable photo. The skilled photographer adds lights to the scene in a subtle way that can help the camera capture more while not changing the mood of the image. We need to remember that Lux is a camera, not an eye. Cameras have a much more limited perception of light than the human eye.

Help the camera and you will get better results faster.

Paolo

https://www.preta3d.com
FB: https://www.facebook.com/RealityPlugIn
Tw: @preta3d
G+: https://plus.google.com/106625816153304163119
The Reality Gallery: https://reality-plug-in.deviantart.com