Bonjour Louis-Philippe :)
That's a cool micro-benchmark. I'm not sure if recursive arithmetic algorithms really represent the real world, but it's cool anyways.
If you remove the startup times from the final number you should see better results for MacRuby. But I agree that the current MacRuby startup time, which is about 0.1s, is bad, and should be fixed (if you file a ticket that would be great).
I recommend to give it a try with MacRuby trunk (you can grab a nightly build) and set up the VM_OPT_LEVEL environment variable to 3.
$ VM_OPT_LEVEL=3 macruby test.rb
You should get good results this way. This will enable optimized codegen and primitives inlining.
Laurent