Thanks Ulco.
There are 13 cloud layers with big image files like some 10 000x10 000 or smaller ones and 8 different 6 000x6 000 image sequences.
It takes around 1 hour for every frame. Detail 3 and AA is 12.
I am sick of noise and used these settings to be absolutely sure. Size is 1920x960.
I think it is actually very fast rendering with those settings and all.
A planet approach could be made depending on the scene but these structures are very sharp and steep and the camera begins a little closer.
Ulco you know those kind of displacements are a little more problematic so i found a cloud approach easier in that aspect.
Beside that, i wanted to see an animated version of my old scene
Edit: I should add that the most important factor for fast render time in this scene is the "Step optimisation" option in the "Quality" tab of the cloud nodes probably. It is 20 (you can use higher numbers too as in many other parts in Terragen) in all layers. But the look of the clouds can get very sliced and may or may not work everywhere.