I was only running the Mesa drivers on the WX4100; I have several system with AMD gpus that work perfectly fine on the mesa drivers, so I don't believe it was simply a driver configuration issue. libva-mesa-driver was installed, but I don't expect that particular library to have changed much as this was 3D rendering rather than video/2D playback. I've never bothered with the closed-source drivers for AMD, at least not since they were named catalystMay be dumb questions and things you already checked: Is Arch loading libva-mesa-driver with the AMD card? I think the system will run Mesa without it (its listed as optional for Mesa), but the framerate will probably be awful, so may be something to check? Are you using AMDGPU or AMDGPU PRO? I've found that AMDGPU PRO actually made my system perform a lot worse. My system differs from yours quite a bit though, so YMMV.
Did you load the proprietary NVIDIA driver package? Current generation NVIDIA driver configures itself quite nicely, but AMD's driver, even the proprietary "PRO" one, sometimes requires some tweaking in my limited experience with Arch.
Did you try any other benchmarks with the AMD? I know the Lexa architecture tanks pretty hard in some of the GFXBench tests for example, maybe Uingine Heaven is hard on it too?
Yeah for the nvidia test it was using the latest nvidia closed-source driver in the Arch repos.
I tried one or two other 3D loads, and they all experienced the weird frame-delay/rubberbanding on the 4100 in this system. I'll try the card in another rig to make sure it hasn't developed any hardware issues.