I have an iMac Pro 3GZ 10 core, 64 gigs of RAM and Radeon pro VEGA 64x 16GB, not the same machine, but mine runs great. I also always use SwitchRezX. I know high DPI displays look pretty and all, but to get high DPI you are literally rendering your display twice, using up twice the ram it should, so I run my iMac display and external display at 2560x1400 to save on Video Ram and it runs great.
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