Swift version: 5.1
Whole module optimization is a compiler pass that can add significant performance gains, and so it's always worth enabling when doing a release build of your app for the App Store. How it works is quite simple: when Swift builds the final version of your app it combines all your source files together and can evaluate the whole structure of your program at once. This lets it make extra optimizations that would be impossible before, when every file was optimized individually.
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Available from iOS 7.0
This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.