From Martin's Computer Programming Blog.
What we can see it that Java itself accounts for 0.35s of the startup time, but unfortunately Clojure adds another second(!) on top of that. This 1.3s pause before main gets called is why Clojure is unsuitable for "terminal scripts". The running time of any scripts (like lein or starting the REPL) will be totally dominated by the startup time. Most Clojure developers will not notice this too much to, since they spend almost all their time in the REPL itself, but users of those Clojure programs will!
My summary is that it spends 95% of the startup-time loading the clojure.core namespace (the clojure.lang.RT class in particular) and filling out all the metadata/docstrings etc for the methods. This process stresses the GC quite a bit, some 130k objects are allocated and 90k free-d during multiple invokes of the GC (3-6 times), the building up of meta data is one big source of this massive object churn.