But why does your cube with no displacement take so long?
I think it's mostly due to size and quality settings I used because if I render at 800 X 450 as you did, and use detail .4 AA4, the plain cube with a totally flat planet terrain and 16 atmosphere samples and no prepass the cube renders in 31 seconds.
Other thoughts.
1. My example renders are 1600 x 900 and yours are 800 x 450, so the bigger has 4 times the pixels to calculate. It should take at least 4 times longer just based on size.
2. My machine is a Window 8, asus I7 desktop with 12 gb memory - 8 threads possible
3. My scene includes a large amount of planet terrain surface that has displacement and shadows that have to be calculated where your example scene has no planet terrain.
4. I have my Atmosphere quality set at 64 samples.
5. I used large diameter soft shadows. No reason, just tried it for looks.
6. I used a detailed supersample GI prepass with settings 4/5/8 - the prepass alone can take 5 minutes.