title: How to modify the priority of a custom GCD queue?
body: <p>I've created a GCD queue like this:</p>
  
  <pre><code>dispatch_queue_t q = dispatch_queue_create("com.testcompany.myqueue", NULL);
  </code></pre>
  
  <p>When I dispatch tasks to that queue, it is way slower than simply executing the task on the main thread.</p>
  
  <pre><code>dispatch_async(q, ^(void) {
      [self performHeavyCalculationAndUpdateUI];
  });
  </code></pre>
  
  <p>My suspicion is that the queue has a very low priority by default.
  How can I change the priority of this queue? Or is there something else I must do?</p>