Nope. Although JRuby is not very efficient in calling Java code, at least it's not as efficient as Clojure is from what I've read.

Another plus for Clojure is that it allows us to easily have a multi-threaded implementation. JRuby allows native threads too, but Clojure make concurrency really easy, while JRuby just wraps Java threads with nothing more, so I would have to use regular locking and stuff.