kotlin-tooling-native-build-performance

Kotlin/Native Build Performance

Turn "the iOS build is slow" into a measured diagnosis and a small set of safe fixes. Two rules apply throughout:

1. Never trade away required release behavior. A faster local loop must not change what CI publishes.
2. Measure before and after with the same command and the same build state. An unmeasured fix is a guess.

Step 0: Classify the Slow Scenario

Establish four facts before editing anything: where (local or CI), what (debug feedback loop or release/distribution artifact), state (first build, clean, warm, or no-op), and phase (which tasks dominate the log). Then match the dominant symptom:

Symptom in the build log	Likely cause	Read
linkRelease* or *ReleaseXCFramework tasks in a local development loop	Building distribution artifacts for development	artifacts-and-targets
Kotlin/Native compiler distribution downloaded on every CI run	~/.konan not preserved between runs	caching-and-gradle
Long pause before the first task starts	Configuration phase, no configuration cache	caching-and-gradle
All iOS targets build when only one simulator is needed	Broad task (build, assemble, assemble*XCFramework) or unused targets	artifacts-and-targets
ksp* tasks ahead of compileKotlinIos*	Generated-code work on the native path	exports-and-generated-code
Small source edit recompiles and relinks everything	Compiler caches disabled, or missing incrementality	caching-and-gradle, experimental
Machine overloaded while several link* tasks run at once	Parallel native linking	caching-and-gradle, worker-limit caveat

Step 1: Audit and Measure

1. Run the static audit from the project root:

   scripts/audit-native-build.sh /path/to/project
   

   It is read-only and prints file:line findings (disabled caches, broad local tasks, transitiveExport, broad KSP configuration, missing CI .konan cache), each pointing at the reference file with the fix. Findings are leads, not verdicts — confirm each against project policy.

2. Find the command the user actually waits for: a script, a CI step, or the Gradle invocation inside an Xcode build phase. Optimize that command, not a task you picked yourself.

3. Run it twice when practical. The first build downloads Kotlin/Native components and fills caches; only the second and later runs are representative. Attribute time per task before blaming the compiler:

   kotlin.build.report.output=file   # writes build/reports/kotlin-build/
   

   Gradle's --scan or --profile work too.

4. If you cannot run the build (no macOS host, no Xcode), analyze logs, build scans, or checked-in metrics instead — and state explicitly that the conclusion is static.

Step 2: Fix in Safe Order

Apply fixes one at a time, re-measuring as you go:

1. Restore healthy defaults — remove cache/daemon workarounds, enable Gradle build and configuration caches, keep ~/.konan warm in CI, update Kotlin: references/caching-and-gradle.md
2. Build only what the feedback loop needs — one specific task per loop, correct integration method, justified target matrix: references/artifacts-and-targets.md
3. Cut export and generated-code cost — drop transitiveExport, narrow export(...), scope KSP work to the native compilations that need it: references/exports-and-generated-code.md
4. Experimental switches last, with the user's agreement: references/experimental.md

Worked Example

A developer on an Apple Silicon Mac complains that "every shared-module change costs 12 minutes". Their loop runs ./gradlew :shared:assembleXCFramework. A build scan of the second (warm) run shows:

:shared:linkReleaseFrameworkIosArm64             348s
:shared:linkReleaseFrameworkIosX64               341s
:shared:compileKotlinIosX64                       96s
:shared:linkDebugFrameworkIosSimulatorArm64       41s
:shared:compileKotlinIosSimulatorArm64            38s
configuration phase                               64s

Reasoning chain:

• The loop is local + debug + warm, but ~690s goes to linkRelease* — release linking is an order of magnitude slower than debug and only CI needs it. Replace the local command with :shared:linkDebugFrameworkIosSimulatorArm64 (or the Xcode embed task if Xcode drives the build). (artifacts-and-targets)
• All iosX64 work serves Intel simulators; ask whether the team still supports them before removing the target. (artifacts-and-targets)
• 64s of configuration on every run disappears behind org.gradle.configuration-cache=true once trialed. (caching-and-gradle)
• Expected loop after the change: ~40s compile + ~40s link on warm builds — confirm by re-running the new command twice and comparing.
• CI keeps assembleXCFramework untouched; note that explicitly in the report.

Verify

• Re-run the exact baseline command; compare warm build against warm build, not warm against cold.
• Second run with the configuration cache reports it is being reused.
• The local development log no longer contains linkRelease*, *ReleaseXCFramework, or removed generator tasks.
• CI still produces every required release artifact, unchanged.
• Tests pass and the app still runs from Xcode.
• scripts/audit-native-build.sh reports no findings you have not consciously accepted and documented.

Report Your Changes

Close with a short performance note:

• The slow scenario (local/CI, debug/release, cold/warm) and the measured evidence — or a statement that the analysis was static.
• Each change, and why it is safe for release behavior.
• The before/after commands the user can run to confirm the win.
• Remaining tradeoffs: experimental flags enabled, targets removed under a policy assumption, worker limits, or generated-code work deferred.
• Links to the relevant official documentation below.

Official Documentation

Topic	Link
Improving Kotlin/Native compilation time	https://kotlinlang.org/docs/native-improving-compilation-time.html
Kotlin Gradle plugin compilation and caches	https://kotlinlang.org/docs/gradle-compilation-and-caches.html
iOS integration methods	https://kotlinlang.org/docs/multiplatform-ios-integration-overview.html
Direct integration with Xcode	https://kotlinlang.org/docs/multiplatform/multiplatform-direct-integration.html
Building final native binaries and XCFrameworks	https://kotlinlang.org/docs/multiplatform/multiplatform-build-native-binaries.html
Kotlin/Native binary options	https://kotlinlang.org/docs/native-binary-options.html
KSP with Kotlin Multiplatform	https://kotlinlang.org/docs/ksp-multiplatform.html