That's an incredibly broad question, so it's pretty hard to answer. Here's some general ones:

* More instances will give you more requests/second, but only so long as secondary system (database, search server, whatever) can keep up with the load

* N+1 queries will kill your performance. They can add a lot of time to processing requests.

* Everything in the most used parts of your site should hit an index on the database. If it's not I can almost guarantee it will be slow.

* Look for work that you can move "offline". As long as they go out within a minute nobody will notice that you aren't sending emails at the exact time they are being generated. Move that work to a background queue so your web server can just add a queue entry and get on to the next request.

* Cache anything you reasonably can. It's a lot cheaper (i.e. faster) to read from a cache than to do a database lookup. It's cheaper to read from a cache than to render an ERB template. Caching can introduce its own difficulties, but the performance gains can be worth it.

Without knowing some specifics of your implementation I really can't give you better advice. Depending on the specific libraries you use it might not even be possible to move to JRuby due to support for C extensions. You might get some performance benefits out of it, but I'd expect it to only buy you some time to execute on some of the above improvements.