In your priority setting you force darktable to prefer the intel on-cpu gpu over the dedicated gpu. Not the best idea if performance counts 
to speed up processing you might use different priorities for full and preview pixel pipe especially when doing previews with reduced resolution
unfortunately to get best results you need to read and understand:
https://darktable-org.github.io/dtdocs/en/special-topics/opencl/
and
https://darktable-org.github.io/dtdocs/en/special-topics/mem-performance/