How do you speed up and diagnose slow builds: incremental builds, whole-module optimisation, explicit modules and the build timeline?

First measure. Build time has distinct parts: dependency resolution, compiling each module’s sources, linking, code signing, asset and resource processing, and run-script phases. Xcode’s Build Timing Summary (Product ▸ Perform Action ▸ Build with Timing Summary), the build log’s timeline, and the compiler flags -Xfrontend -warn-long-function-bodies=100 and -warn-long-expression-type-checking=100 (which warn on bodies or expressions that take more than that many milliseconds to type-check) show where the time goes. Typical culprits: a few pathological expressions (long operator chains, big literal arrays and dictionaries without type annotations, heavy result-builder bodies), too few or too many modules, a shared hub module that invalidates everything, a slow script phase that runs every build, and clean builds on CI with no caching.

The levers. Incremental builds: Debug compiles only the files affected by a change (the compiler tracks dependencies between declarations); keep them effective by making interfaces small and stable, avoiding huge files and avoiding changes to widely used headers or “core” declarations. Whole-module optimisation (WMO), used in Release, compiles a module as one unit for better cross-file optimisation (specialisation and devirtualisation) at the cost of recompiling the whole module on any change, so Debug stays incremental and per-file. Modularisation lets unchanged modules skip work, but only if the graph is shallow and hubs are stable (see the module graph question). Explicit modules (Xcode 16: a dependency-scanning step that builds module dependencies as separate, cacheable tasks) make builds more parallel and deterministic. Compilation caching (a content-addressed cache shared between machines, such as the compiler cache in recent Xcode or distributed caches in Bazel and Tuist) avoids repeating identical work on CI. Static linking of internal modules, fewer dynamic frameworks, and mergeable libraries reduce link time and launch cost. Previews slow down when the previewed target depends on a large graph: give features a small preview host or demo app. Also: type-annotate to help the type checker, split giant files and expressions, turn off unneeded debug-info generation for dependencies (DEBUG_INFORMATION_FORMAT), keep script phases incremental by declaring inputs and outputs, and use a fast machine and SSD. Change one thing at a time and record before and after, because build-time folklore is full of tips that do nothing on your project.