Thank you Jay
The clouds have edge sharpness 16 and density of 0.0025 and the quality setting is 1.46195 which results in just 48 samples.
Therefore I think the clouds do not add that much more rendertime, but maybe I should check that to be sure.
The reason for the somewhat long rendertime is the crazy detailed rockstructure (some redirects, strata, voronoi etc.). Together with GI 2/2 and detail @ 1 it is quite demanding.
I might decrease rockstructure complexity, since the majority is in the shadow, but you'll never know for future ideas so I'm happy it's already there
Martin