Preview2 is only relevant if you use a 2nd window for preview.
Mine is set to
opencl_device_priority=+0/+0/+0/+0/+0
to run everything on the GPU.
darktable does fall back onto the CPU if the GPU does not complete the computation quickly enough. The timeout is defined by
opencl_mandatory_timeout=20000
That’s not the default value; I increased it so darktable does not fall back to the CPU too quickly (with large images, diffuse or sharpen can take time, but falling back to the CPU would mean much longer processing times):
time period (in units of 5ms) after which we give up try-locking an opencl device for mandatory use. defaults to 400 (2 seconds).