How do you reduce launch time? What is pre-main?

Senior PRO

The point

Launch has two phases:

• Pre-main — everything before main() is called: the kernel creates the process, dyld loads the executable and dynamic libraries, applies fixups (since iOS 15 chained fixups instead of separate rebase/bind — faster and lazier), registers Obj-C classes and categories, runs +load and C++ static initializers. Your code barely participates here — only +load and global initializers.
• Post-main — UIApplicationMain, didFinishLaunching, scene creation, the first layoutSubviews/body and the first frame. Apple targets a first frame within 400 ms (cold start).

Separately: prewarming (iOS 15+) — the system may launch the process in advance and pause it after pre-main, so pre-main is already paid for at the "real" launch. This matters for measurement: the process uptime from ProcessInfo is not the time the user saw.