Thank you, I didn't know about that.
For those interested in learning more, the pull request for the (still experimental?) setting is here:
A new SIL module pass "annotates" functions and types with @inlinable and @usableFromInline.
A comment here (by the author of the pull request) says that it can also be enabled using the undocumented Xcode build setting SWIFT_CROSS_MODULE_OPTIMIZATION.
I'm not sure about the impact on runtime performance yet, but it definitely has a big impact on buildtime performance. :-)