I isolated and detached all the stone poly visible from camera and applied displace only on them instead of the whole object.
I also passed the displacement screen size from 2px to 4.
Now the render takes around 3h30 to finish at 2% Noise Level.
The easiest way to check if the issue is related to displacement is disabling it. It can be done in the Common tab using the “displacement” checkbox.
2px displacement size may be overkill for 4k resolution.
Sorry for no replies in a long time…
What was the final result? Did you manage to render this?
If there are still problems with this scene (also in Corona Renderer 1.6), we are interested in inspecting it.
The px size for displacement is really easy - it is simply the allowed size for the smallest detail. No feature of the displaced geometry will be smaller than this value. You should use the highest value which looks acceptable, and also it is a good idea to use bump for the smaller details.