I disagree that the slow part of a RoR app is the database - from my anecdotal experience, the slowest part is rendering views and the string manipulation there-in, probalby caused by GC. For a Rails 2.3 App with REE and MySQL, time is ~25% GC, ~25% DB and 50% ruby (from NewRelic).

However, premature optimization and all that, don't choose Java because its faster than Ruby in some arbitrary benchmark. Choose Ruby because it makes you happy and you can meet business objectives faster and easier with it.

On the other hand, if you put your DB on SSDs, back to square one.