Xcode saying “Compiling Swift” when your change was trivial? In mixed Swift/Obj‑C workspaces, that’s often swiftc and Clang reparsing broad header trees on every file. Make modules explicit and you prebuild once, then reuse.

• Shift work upfront: explicit modules compute the dependency graph and prebuild modules deterministically, turning random rebuilds into predictable reuse across targets.
• Rollout safely: start with leaf frameworks, enable in Release/CI first, then Debug; set Build with Explicit Module and add -explicit-module-build (or SwiftPM’s .unsafeFlags for selected targets).
• Fix headers: keep umbrella headers in‑module only, author module.modulemap where needed, and reproduce CI issues locally with xcodebuild and matching HEADER_SEARCH_PATHS.
• Stabilize caches: pin the toolchain, keep a stable DerivedData location, and track timing with xcodebuild -showBuildTimingSummary; instrument custom steps using OSSignposter for clear boundaries.

Choose explicit modules when targets expose deep C/Objective‑C surfaces or are widely reused; keep implicit builds for tiny Swift‑only targets