The specific optimization this achieves is during build time only: these new files are static libraries that are quicker to link. It is a small shift of some of the linking pipeline into the (parallel) builds of dependent libraries, rather than heaping it all onto the linker at the end of the build. The linker has to essentially re-link from scratch for every small change.

Parallelization has long been known as the best way to speed up linking. This Apple change comes in addition to rewriting their linker to be massively parallel. Mold did it first, though: https://github.com/rui314/mold

edit: lld did it first, mold improved upon it

A faster development cycle is one of the coolest improvements that Zig brings – lightning fast recompiles, bringing the native development cycle closer to web speed. More languages should get this capability, and Apple needs it for Swift to get instantly updating SwiftUI previews.

Static linking is required to get the best performance: no linker runtime cost, cross-language LTO and dead code elimination

If this optimization is generally applicable and developers find it worthwhile, I could imagine this making its way to GCC-land. At the very least, gold needs replacing/rewriting to be competitive now.

edited: for clarity