Even so the point of the post is not to bash on Leiningen but to demonstrate how the overhead issues are not insurmountable - it just requires really knowing the tools (surprise) and which knobs to turn. In fact an implicit free idea for someone to explore is to automate all the optimizations for Leiningen that I covered.
As for cljsbuild, I'm guessing this is more a clojurescript compiler issue since again, I run leiningen once and leave it to recompile on file change in the background. Yes would be nice to have some compiler speed improvements but really it isn't that bad. I wait a second or two, not tens of seconds. At least, this is the case for me but then my cljs projects tend too be a couple of thousand lines at most.
There are JVMs with JIT caches and AOT compilers.
Even the reference JDK can start in a few ms using tiered compilation.