Kotlin Compiler Plugin Cuts Android Startup Time by 30% in Expo SDK 56

Expo SDK 56 ships with a custom Kotlin compiler plugin that eliminates reflection from Expo Modules on Android. The result: 70% faster module initialization and a 30% reduction in time to first render. The plugin runs during compilation, so app developers get these performance gains automatically without changing any code. Module authors can unlock even bigger wins with a single annotation. This post walks through how we built it and why this approach succeeded where previous attempts failed. For the Swift side where we now talk to JSI directly, check out our companion post Talking to JSI in Swift . The reflection problem we inherited Before Expo Modules, we had Unimodules. They worked like old React Native bridge modules: you'd sprinkle annotations across methods you wanted to expose, and the runtime would discover everything through reflection. class ClipboardModule ( context : Context ) : ExportedModule ( context ) { override fun getName () = "ExpoClipboard" @ExpoMethod fun getStringAsync ( promise : Promise ) { val clip = clipboardManager . primaryClip ?. getItemAt ( 0 ) promise . resolve ( clip ?. text ?. toString () ?: "" ) } @ExpoMethod fun setStringAsync ( content : String , promise : Promise ) { clipboardManager . setPrimaryClip ( ClipData . newPlainText ( null , content )) promise . resolve ( true ) } } Reflection made sense when we needed metadata about our own code. What methods does this module export? What arguments do they accept? The JVM could answer those questions. But reflection costs time, and on Android that time comes straight out of your startup budget. Every module the runtime introspects adds milliseconds before users see your app. Building the Expo Modules API gave us a chance to fix this. We wanted better ergonomics and less reflection. The Kotlin DSL delivered both in one move, removing most reflection while making modules easier to write. But we couldn't eliminate all of it. Type information for function arguments and Record properties s