And dispatch_async loses QoS while dispatch_sync keeps it, but people often used to async unnecessarily.

>dispatch_async loses QoS

Wait really? Where can I read more about this, this goes against what I would assume.

Edit: Or do you mean that when dispatch_async the block is run with the QoS of target queue instead of source? That is what I would normally expect, if you want to "inherit priority" then dispatch_sync would do that, at the expense of blocking.