At least with the previous JRuby, for us the answer is "usually no." If you do any large string ops (hello JSON), or parsing, or use any ruby gems which use exception handling for flow control (which is cheap in MRI, but like 9ms per exception in JRuby) in a loop then you will be slower. Also, SSL connection negotiation is exceedingly slow in JRuby 1.7x (like 100ms) and this made our microservices tough. Using HTTP Client with keepalives helped with this.
In a hello world, you might come out ahead in 1.7.x. Maybe 9000 is better?
I wrote Manticore (https://github.com/cheald/manticore) because of my dissatisfaction with Ruby HTTP clients under JRuby; it uses the Apache HTTPComponents (completely sidestepping Ruby's stdlib http/networking stuff) and is extremely fast. It might be worth looking into for you.
Do you have anything you can point at regarding String performance? Everything I've seen shows JRuby being faster than MRI. But I'm looking to see if we can make it even faster than it currently is. Any pathological cases would be incredibly helpful.
I can send you an example. Can I drop you an email? I think also that we are friends on linkedin, coincidentally. I can message you there (if that is a useful way to get in touch?).
In a hello world, you might come out ahead in 1.7.x. Maybe 9000 is better?