Best practices
Show something as soon as possible.
Let people do other things in your app or game while they wait for content to load.
If loading takes an unavoidably long time, give people something interesting to view while they wait.
Improve installation and launch time by downloading large assets in the background.
Showing progress
Clearly communicate that content is loading and how long it might take to complete.
For games, consider creating a custom loading view.
Platform considerations
watchOS
As much as possible, avoid showing a loading indicator in your watchOS experience.
For the complete guidance, including worked examples and illustrations, see the canonical page: https://developer.apple.com/design/human-interface-guidelines/loading