The issue is, to prevent concurrent writes of the same Vulkan descriptor we must delay writing them until command list submission, so instead of descriptors being written on the fly, often by multiple threads, we write them all at the last millisecond from a single thread.
Are concurrent writes to the same descriptor really allowed in DX12? If so, wouldn't that mean every DX12 driver has to use atomics/locks to update its descriptors? That seems like a lot of burden to put on drivers for little benefit to games (since they'd still be unsure of which descriptor write overwrote which). Are you sure this isn't a leftover from pre-update-after-bind, where entire descriptor sets needed synchronization, in which case we should be able to update concurrently as long as update after bind is used?