From the narrow scope of web development, JVM is fine. Kotlin is able to have every ergonomic feature you'd want from a language, and it gets closer to the performance of C in TechEmpower than many other languages (notably C#, Node, and Python, three of the most popular languages).
Tuning can be a pain, but scaling anything is hard.
It is clear that a lot of very important and practical work had been delivered with Java/JVM. But the original point was not about what is possible, but what is optimal.
Kotlin, from the little I've seen, appears to be largely a syntactic variation of Java, so there is certainly good impedance to JVM. Clojure is less so, and here lies the reason everyone curses about its backtraces.
Tuning can be a pain, but scaling anything is hard.