Why does it take such a long time for UI to be updated from background thread?

Question

I understand that all UI updates must be done from Main thread.

But purely for the sake of deeper understanding how GCD and dispatch main work:

I have a butt

Answer 1

If you attempt to do UI updates from a background thread, "the results are undefined." The most common effect I've seen is what you describe - very long delays before the update shows up. The second-most common effect I've seen is a crash. The third-most common effect is some sort of drawing artifact.

The results of doing UI updates from a background thread are truly nondeterministic. You've got multiple processor cores accessing the same hardware resources at the same time, and with the exact timing between those accesses being unknowable and infinitely variable. It would be like having a computer with no display but 2 keyboards and 2 mice, and 2 operators editing the same document at the same time. Each person's actions with the keyboard would change the state of the document, and screw up the changes the other person was trying to apply. The cursor would be in the wrong place. The amount of text in the document would be different than expected. The scroll position would be off. etc, etc.

Similarly, if 2 cores are each trying to access hardware resources to do screen refreshes, those accesses will cross and conflict with each other.

As Martin says in his comment, the UIKit code is proprietary, so we can't know the details of what goes wrong. All we know is that bad things happen, so DON'T DO THAT.