There are threading Problems in Blender 2.8, when using CPU + GPU
This video shows an example rendering without the treading problem. It takes a little over 3 minutes.
This is an example with the threading problem. The calculation of nearly the same scene (only the main characters moved a very little bit) and with exactly the same Blender 2.8 settings it takes over 5 minutes. The GPU usage goes up to nearly 100% and the GPU is not faster then the CPU cores anymore
I had the computer running over night. As i looked at it next morning it was with the threading problem. I just closed Blender, started it again, load exactly the same project, start it and it was much faster again.
I filmed it with my smartphone camera, because when the threading problem occurs and I try to make a video with windows apps, the problem disappear when I start the recording. This is an additional hint that it is a threading problem.
PLEASE do something about it !!!
If you need any information e.g. about used options … just ask I will help as much as possible!