How we sped up Android and iOS builds by 68% on CI

Duolingo's engineers were stuck waiting 50 minutes for every CI build. That's hundreds of hours wasted every week across the team.

The Test and Release Infrastructure team tackled this head-on, cutting Android and iOS build times by 68% through systematic experimentation. They tested everything from hardware upgrades to caching strategies, documenting what worked and what flopped.

Key Takeaways

• Upgraded to AWS r7a instances for Android (21% faster) and M2 Macs for iOS (60% faster)
• Gradle remote S3 caching eliminated redundant compilations, saving 25% on Android builds
• Switching from KAPT to KSP plus Kotlin upgrades delivered another 30% improvement
• SwiftPM dependency caching shaved 4 minutes off every iOS build
• Pipeline parallelization reduced Android serial execution time by 20%

They also share why XCRemoteCache failed spectacularly after months of effort, and the surprising lesson about why more CPU doesn't help Android builds.