GTX1060 and Threadripper 1950x. 64GB ram. Ubuntu 18.04.
GPU: 5,886015 [dev_process_export] pixel pipeline processing took 5,206 secs (49,141 CPU)
CPU: 9,813531 [dev_process_export] pixel pipeline processing took 9,248 secs (242,660 CPU)
I changed opencl_memory_headroom=300 to 1200 and got:
5,198685 [dev_process_export] pixel pipeline processing took 4,478 secs (22,712 CPU)