I revisited an old scene, had to make a few changes in objects (as some were deleted), and rendered in PT. It too 6hours 20mins, and my processors kept so cool I had to check what happened. Seemed processor use (all 8 cores) was only 24-25% all the time. How is that possible? Build 4.4.18.1 was used. Could it have been because it was an old file with some values baked into it?