I think, in a lot of cases when people still complain about "the slow startup time of the JVM", they're really just talking about how the big JVM GUI apps (like IDEs) struggle to get started on heavily-loaded systems. And this, I think, is mostly just due to these apps eagerly reloading the most recent workspace on startup, and so pre-allocating big chunks of memory on startup to be ready for that — which can ripple out, on systems with low free memory, as other, colder processes all bottlenecking together on the IO of having their own memory written out to swap; and/or to having dirty mmaped pages forcibly-flushed to disk
en masse so that the page-cache entries they live in can be purged.
Much more rarely — mostly when talking about writing CLI tools in a JVM language — people actually are complaining about the single second-or-so it takes the JVM to start up. (Usually because they want to run this tool in a loop under xargs(1) or find(1) or something.)
This last second of startup lag is (AFAIK) quite hard to improve, as it's mostly not the JVM itself starting up, but the static methods of JVM classes being called as those classes are loaded — which can do arbitrarily much stuff before any of your own code gets control. (Due to legacy code expecting to read certain per-boot-dynamic info as static fields rather than as the results of static method calls, I believe the JRE runtime is actually required to do quite a lot of that kind of static initialization, to pre-populate all those static fields, just in case something wants to read them.)
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You'd think that GraalVM could inherently skip most of this, because the Graal compiler does dead-code analysis. "If nothing in your code reads one of those static fields, then nothing needs to write that field, so let's not invoke the static initializer for that field." But that's not true: static initializers are exported and called by the runtime — so they're always "alive" from the compiler's perspective. The Graal compiler isn't doing full-bore data-flow analysis to determine which static members of which classes are ever read from.
I believe GraalVM does try to work around static initializers as much as it can, by pre-evaluating and snapshotting as much of JVM runtime's static initializer code as possible by default, converting it all into const data structures embedded in the class files before the native codegen step gets run on it (see: https://www.graalvm.org/latest/reference-manual/native-image...).
It's not possible to apply this pre-baking to 100% of classes, sadly — some of these data structures need environment data like env-vars or system network config threaded into them on boot.
(I wonder anyone on the Graal project is working on a fully-general static-initializer optimization pass, that does something like concolic execution to bake these initializers out into either fullly-constant data if possible, or if not, then constant "data templates" plus trivial initializer functions that just gather up the few at-boot context fragments, shove them into the "data template" using a low-level hook, and then memcpy the result out onto the heap and call it an object.)