Your understanding is correct. JVM startup accounts for a small portion of Clojure's startup time.[1] Most of the overhead is in compiling and loading clojure.core. Efforts have been made to remedy this issue[2] (e.g. direct linking, ahead-of-time compilation, ...), but this doesn't remove the fact that clojure.core is huge and virtually any Clojure program will be importing more than just clojure.core, so there is still a lot of var derefs to deal with, then code to compile, then classes to load, etc.
Well, it's still partially the JVM at play. For example, if your application has big classes, and many classes, the JVM will be slow to start. This is what is happening here. Clojure is like a large-ish Java project, it has big classes, and many of them, with static initializers, that need loading at the start, and the JVM does all that slowly.
In some sense it's Clojure's fault for having an implementation that causes slow JVM startup, but it's also the JVM's fault that the way Clojure uses it causes it to take a long time to start.
Finally, although technically not really Java nor JVM, the Android Runtime (ART), does a mix of high performance interpreter written in Assembly, JIT, AOT compilation, and PGO sharing across devices via Play Store (cloud profiles).
Well, quite a few people already use AOT with PGO from GraalVM to build native executables of Clojure programs. Those start stupidly fast. I never heard of anyone doing so with OpenJ9, how good is the AOT of OpenJ9?
AppCDs in OpenJDK currently has terrible ergonomics. Clojure can't really offer it. Each user must go out of their way to leverage it. So you can't really release an app that automatically leverage it, the user needs to launch it with all the command incantations, etc. And it's so sensitive to class path changes, etc. It kind of sucks to be honest. But some people still use it for prod release, since you can set it up in a docker easily. But the use-case for fast startup are desktop apps, CLIs, scripts, etc. And for all those, AppCDs are super annoying to setup. See: https://ask.clojure.org/index.php/8353/can-we-use-appcds-to-...
Still, AppCDs don't fully solve the startup issue, because all the static initializations stuff takes a considerable amount of time, and that does not get cached by AppCDs.
