Swift's unpredictable efficiency

Yeah, sorry, "Cross-Module Inlining" is an unfortunate title for a set of features that are designed to enable a resilient standard library. They weren't design as a way for regular developers (who don't care about resilience) to split their app into modules. For now, you're better off not splitting up the app if you discover that runtime performance is a problem. The standard library's performance issues are self-inflicted by building with resilience enabled. So the features described in those documents are super important, but not precisely relevant to this thread.