Lisps in general are appealing to implement. Clojure's core language is about as small as Scheme's, and there are a lot of Scheme implementations too. The parts of Clojure that set it apart from other Lisps (STM, persistent data structures) aren't always ported. I don't think any of the recent approaches to targeting C (Clj->Scheme->C, Clj->C) include STM or persistent data structures. I'd be happy to be pleasantly surprised though.
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...