Your JVM does. jruby.jar is megabytes of reimplementation and mapping of the Ruby stdlib onto the JVM. Plus in this example, loading a Ruby parser and interpreter.
Here's executing an assembly (über-jar) I wrote in Scala:
→ time scripts/couch
Please provide a basePath and a valid command. ("backup", "restore", "truncate", "migrate" or "migrate-all")
real 0m0.375s
user 0m0.415s
sys 0m0.073s
That's basically just `java -jar couch-utils.jar`.
A few hundred ms on my 1.2GHz MacBook. And this includes a bunch of Akka Actors.