How can I set high priority to a custom created serial asynchronous queue using GCD's?
I had a look at this Q&A where suggestion is made to make use of
Your queue will still be serial. It will just be performing its tasks, one at a time, in one slot of the high priority, global, concurrent background queue. Once created, a serial queue cannot be "made concurrent" by any means.
Similarly, if you create a concurrent queue and you set it to target a serial queue, it effectively becomes serial.
This is all covered in this man page.