radv is one of the ones I'm referring to, it's open source so we can see the tricks they use. One example: it spawns a thread behind your back in order to execute command buffers in order to make the queue submit appear nonblocking (or at least fast). This is completely contrary to the spirit of the Vulkan spec which deliberately doesn't provide any convenience mechanisms for things like callbacks on command buffer completion, precisely because these would require runtime threads to be around. The user has no control over this thread, including where it spawns or its priority (how could it, when it's an implementation detail of the driver?) nor can they take advantage of this knowledge to improve synchronization on the CPU side (e.g. by removing external synchronization around queues).

Contrast this with the situation on Metal (explicit runtime that spawns threads, and explicit callbacks to be able to take advantage of that) or DirectX 12 (explicit runtime that works for the average case, with extremely fine grained control provided over the threading model, priorities, etc., including being able to turn them off completely if you really need it). Both of these are clearly much better models because they are actually exposing the detail that seems obvious in hindsight (applications need queue submission to be nonblocking) and are able to provide much more flexible, efficient, and useful APIs as a result.

By leaving everything up to the user, Vulkan in practice ends up underperforming its competitors unless the drivers patch up the work--at which point you have the unfortunate situation where you have people targeting a low-level API running on a high-level runtime. It's this, not some dumb digression about extensions, that IMO make Vulkan kind of disappointing--it's definitely not a failure, but it could've been much better than it was.