Some developments on the Volumetric effects performance, I handed an apitrace over to the dxvk developer and here's what he had to say:
https://github.com/doitsujin/dxvk/issues/929
Thanks for attaching the apitrace.
Ironically I'm getting better performance when replaying your apitrace on my AMD card that you seem to get during gameplay, and I don't see high GPU load either, so it's likely that the Nvidia driver fails to optimize the volumetric lighting shader properly. My Nvidia test system is currently out of order due to a broken hard drive, so I can't test it there right now.
Looking at the particular shader, I'm not even surprised though. Frontier clearly wrote inefficient code, Microsoft's HLSL compiler for some reason also failed to optimize a simple constant array to an "immediate constant buffer" as they call it, and now we have a shader that initializes a fairly large constant array in every single invocation, just to read **one** value from it. I wouldn't be surprised if that's the issue here. DXVK doesn't optimize that (as it doesn't do any code transformations besides a naive dxbc->spirv translation), so the driver has to deal with this mess.
https://github.com/doitsujin/dxvk/issues/929