Assuming you've already bootRun or compiled once and are starting the app again, what's your startup time? I'm currently working on an app where it's taking ~30 seconds to get started and wondering what the lower bound could look like with the proper optimizations. I'm generally happy with Spring Boot + Kotlin but find it hard to cope with the turn around time for updates to take effect. With spring boot dev tools enabled, the hot reloading seems to take about 10 seconds. Might not be all that long objectively or compared to how long it used to take, but it certainly feels slow relative to backends in JS, Python, Ruby, etc.
Have you tried profiling the app? What is taking so long to initialize? You could try creating an empty Spring Boot application to see what the lower bound is. One thing I've found with with DI is that precompiled solutions like Dagger are way faster than something like Spring's DI.
In a code base I once maintained there where PostConstruct/InitializingBean/ApplicationListener bean's that took quite a bit of time making maps for use as caches. This contributed to quite a bit of time. So I would look at any misbehaving beans.