The answer is, if the GPU is powerfull enough you should disable the CPU flag. If the GPU is weak the CPU should be enabled.
In my case I tested the latest 1.6 version in demo mode with laptop i7-8750H and GTX 1050 4GB.
The build arbitary model of 50 photos 20mpx took 1,5 hours to complete with GPU only and the same test with CPU enabled took only 20 minutes!
So you should test it in your configuration to see which is better.