Absolutely not--indeed--just the opposite...;) My RX-590/480 8GB *Crossfire* systems runs like a top--very fast--especially in DX12 multi-GPU titles like Shadow of the Tomb Raider--I get 62 fps all eye-candy on @ 3840x2160 multi-GPU; RX-590 by itself, 33 fps average. Honestly, You couldn't give me a nVidia card these days...;)
Just to eliminate any misunderstandings on my behalf: you're getting 33 fps on average at 2160p with a single RX 590?
I just put everything on the lowest possible settings, 1080p, just to see what gives. Same issues for me: erratic framerate, visual glitches (cloud/sky rendering), ...
I have no idea what's going on. My system is definitely capable of running the latest games: i7 4790S, 16GB DDR3-1600, ...
I reread it and I see I wasn't as clear as I should have been--the 33fps @ 3840x2160 was generated @ 33fps with the in-game ShadowoftTombRaider benchmark running the RX-590 8GB in DX11 mode sans multi-GPU. When I run the SoftTR in-game bench in D3d12 mode with multi-GPU enabled (Crossfire) I get a ~62 fps per second average, with peaks > 100fps. But I'm talking about SoftTR, not NMS--I *assume* that SoftTR is the more demanding game on the GPU--even though I have both games running at 3840x2160. With NMS I can tell you only that I get a smooth, stutter free (mostly) frame rate @ 4k with none of the glitching that you are reporting.
One thing that jumps out at me is your DDR3 Ram--I'm running DDR4 @ 3200MHz; wheras you are running DDR 3 @ 1600MHz...which is quite a difference--my ram is running > than 2x as fast as yours--in both MHz and the the type of system ram you have (DDR4 all things being equal is at least 25% faster than DDR 3, then your ram speed is 1/2 of mine at 1600Mhz. Latencies are higher with DD4 but DDR4 requires less power and runs faster--even including the higher latencies, etc.)
Your cpu is 4/8 cores running at 3.2GHz, with only 8MBs of cache; mine is 6/12 cores with all cores running at 3.8GHz, with 16MBs cache L3, 3MBs L2 cache. Also, I noticed that your cpu comes standard with an Intel® HD Graphics 4600 running at 350Mhz-1.2 GHz.
I will tell you what it sounds like--it sounds like you are running your native Intel GPU *instead* of your add-in RX-590 8GBs. Well, your system is a fair bit slower than mine even without considering the GPU--but if you are running your Intel GPU instead of your 590 that would certainly account for everything you've told me so far. Without a doubt.
Before I bought the RX-590 8GB, I was running my RX-480 8GB by itself @ 3840x2160 and never saw any symptoms like you are describing--so I can can categorically state that your system performance in NMS has *nothing* to do with any RX-590 8GB you have installed...;) So--something else is happening in your system--you have some serious problems, somewhere. But again--what you are describing isn't your RX-590 8GB--unless you have a defective card somehow. But that doesn't seem likely.
Query: do you have your RX-590 connected with the 8-pin PSU adapter properly? If you haven't for some reason plugged the right PSU adapter in or you have too little in the PSU department--that could also cause you problems.
So check out your PSU wattage and amperage, check you PSU connections to your RX-590, and make sure that you aren't running the Integrated Intel GPU in your processor--for starters.
Optimization and Vulkan support are coming and are already available in the Steam Experimental branch. Hopefully this will help a lot of people get better performance, especially you AMD guys.
GOG offers the same Experimental branch--which HG is good enough to update at the same time as they update the Steam experimental branch, I'm glad to say. And my AMD performance is nice at the moment--but if they can deliver more--I'm certainly not going to turn it down...;)
EDIT: Downloading the beta Vulkan build through GOG Experimental Beta channel right now--thanks for the tip--looks like it hit three days ago!