Tuning Performance, Memory Usage, and Multithreading
People want answers. Now. They expect their devices to provide those answers quickly. If they believe the applications on their devices are locked up or even just slow, they’ll abandon or perhaps even delete them. You can show spinners to indicate that your applications are working hard on providing those results, which will mollify users somewhat. Giving a visual clue that the device is working beats giving no such clue and letting customers think their devices have locked up; better still would be to never have episodes of slowness that make users wait. You may not always be able to achieve that goal, but you should always try.
Fetching and storing data in a persistent store ...