O have a weird problem with my RX 5700XT - which replaced my Vega 64 ( which died, sadly ).
The symptom is that moving windows around, or typing text and alike are extremely stuttery - it works like with default Microsoft driver. But latest AMD drivers are installed. 3D games work, most of the time, fine ( there is one that has issues, but it seems the game's fault ).
Mobo: Gigabyte X99-UD4
Cpu: i7 5820k @ 4ghz
Ram: 16 gigs
GPU: Gigabyte RX 5700 XT, reference design
Display is an LG 29UM59 connected through DP->HDMI cable capable of 4k ( so I could get Freesync and 75Hz refresh).
With Vega everything worked fine.
The GPU is inserted into the third PCIe slot, not the topmost. CPUz shows full x16 connectivity and no other expansion cards are there.