iOS Startup Performance in Practice, Part 1: Understanding Launch Types and Entry Points

What happens when an iOS app opens, which lifecycle callbacks are executed, and where startup measurement should begin and end.

An iOS application does not have one launch path.

A person may tap the app icon after restarting their phone, return to an app that is still suspended in memory, open a Universal Link from Safari, select a Home Screen quick action, or tap a notification that points to a particular screen. All of these actions appear to have the same result: the application opens. Internally, however, they can execute very different combinations of system and application code.

This distinction matters whenever we discuss startup performance. If we mix a process launch with a resume, or measure an icon launch while most users arrive through notifications, we can collect an accurate number that describes the wrong experience.

In one of the production applications I worked on, startup time had gradually...

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