The operating system does a lot to attempt to keep things responsive for the user. For example, phones attempt to update the UI 60 to 120 times per second (60 at minimum). There's a short finite time to prepare and draw the UI (at 60 fps, every screen update should take 16ms or less).

Android will drop frames, or abort trying to complete a single update cycle to attempt to catch up. Some frames drop and fluctuation is normal but too many will make your app unresponsive.

Performance issues, race conditions, and hard to reproduce bugs are some of the reasons why we don't recommend working with threads directly.

Use Coroutines for writing the concurrent code.