This is more about the Ruby VM being much slower than the JVM than anything to do with Clojure. It would be somewhat interesting to see how JRuby 1.7 running their Ruby code but using the Java libraries would fare.
JRuby is still going to be slower than clojure, due to the semantics of ruby. All function calls are resolved at runtime, while clojure resolves as many as possible at compile time.
Foo::Bar.method() vs (foo.bar/method)
The ruby version does three loads from the heap, at runtime, while clojure does zero.
invokedynamic in JDK 7 will help narrow the gap, but the fact remains that Clojure was designed with more performance in mind than ruby.
I don't think this is quite right, although I'd be delighted to be corrected. Clojure still has to deref the var in order to call the function, and afaik this happens at runtime (hence the fact that you can redefine a function in the REPL).
