Well our game runs faster in DX12 under Proton than Vulkan under Proton.

Of course since Proton uses Vulkan to implement DX12, it means that our Vulkan implementation is simply worse than the one that Valve created to emulate DX12.

I'm sure it's possible to improve that, but it implies that there way to get the best performance out of Vulkan is less obvious than the way to get it out of DX12.