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.
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.