Using VisionTek Radeon R9 380 2GB GDDR5 card (attached to Dell Monitor by mDP) installed in GigaByte Z170X Gaming 7 motherboard with i7-6700K cpu. Windows 10 Build 14342.1001. Latest Crlmson drivers, 18.104.22.168.
System runs extremely slow, unusable. When monitor attached to onboard Intel Graphics DisplayPort, system zips along. Same problem occurred with previous Windows 10 Builds 14nnn.nnnn.