Why is Micronaut faster at startup than traditional Spring Boot applications?

The gap comes down to when the expensive work happens. Traditional Spring Boot relies heavily on classpath scanning and reflection at startup to discover components, resolve dependency graphs, and build AOP proxies, and all of that costs CPU time and memory on every boot.

Micronaut moves that same work to compile time. Its annotation processor scans your source during the build and emits plain BeanDefinition classes and AOP proxy classes as ordinary bytecode. When the JVM starts, the ApplicationContext just loads and executes those pre-generated classes directly, so there's no classpath walk, no reflective introspection of every class, and no runtime proxy generation via CGLIB or JDK dynamic proxies.

The practical effect is startup times often measured in tens to a couple hundred milliseconds for typical services, versus multiple seconds for a comparable classic Spring Boot app, with a correspondingly smaller memory footprint since there's less metadata to hold in memory.