No doubt - excellent job, C* folks! Don't get me wrong, I'm not trying to be bitter but I'm genuinely curious how much of that improvement can be attributed to JVM (?). And, also, how much of an "optimization reserve" C* itself still has (and will pull off one day)?

More than JVM, it's the new Java Garbage Collectors (GCs) like ZGC. You can read more about their impacts here:

https://www.scylladb.com/2020/10/06/c-scylla-in-battle-royal...