ClojureScript's core library has a dependency on Google Closure, and are optimized with knowledge of javascript vms. david nolen said that v8, for example, is blazingly fast at sequential instructions so they take great care to avoid function calls in critical paths.

here's some discussion that alludes to performance concerns in js vms: http://dev.clojure.org/jira/browse/CLJS-284

here's the original commit for PersistentVector, note how low level the code is: https://github.com/clojure/clojurescript/commit/e615f4cd326e...

here's more discusson by david nolen: https://groups.google.com/forum/?fromgroups#!searchin/clojur...

more discussion about allowing low level code access to js-specific special forms for numerics: https://groups.google.com/forum/?fromgroups#!searchin/clojur...