Spring boot has a large startup time, heavy dependency trail and a lot of dynamic class-loading based off auto-configuration. This makes it un-suitable for some areas: if you desire fast scaling micro-services that respond quickly to incoming load. Or say you want to transform your micro-service to a server-less function. One can do that with Graal VM to compile your micro-service to a single binary with microscopic start-up time. However, native-image tends to fail more often than not with Spring boot apps. (I haven't tried this recently though so my knowledge could be out of date).
Spring Boot does what it's asked to do, which is to load everything it finds. The dynamic class work has basically nil effect on load time. The core performance constraint is that classes are loaded essentially linearly. If you ship with something enormous like Hibernate, that's thousands of files read individually from a zipfile.
Spring folks are actively involved with Graal. I see a lot of excitement internally at Pivotal. Watch this space.
I'm reminded of the crazy stunts from the 1980s where people would start some big, slow program (Emacs was huge!), dump core, and then "undump" to make a pre-initialized binary that would start as fast as the OS could read it. It actually worked, as long as it wasn't relying on open file descriptors, or signal handlers, or env vars, or …
I’m pretty sure Emacs still does this. There was a patch committed to change it to use a more portable method of dumping state, but I don’t think that change made it into the 26.2 release.
