Generally speaking I expect Vulkan to be slower than ROCm given it's designed for generic gaming across GPU vendors, so the takeaway is, whenever ROCm is available and usable, we should use ROCm. And it's the same for CUDA vs Vulkan.

What slows it down? Shouldn't Vulkan expose compute queues of the GPUs as well?

I don't have any expectations, but there're reasons for Vulkan to be faster.

It's a mature technology used my millions of people every day.

Unlike GPGPU compute, for videogames performance directly affects usability.

For these reasons, the software on all levels of the stack might be more optimized.