Stripe tried switching lots of code to the JVM and it was a huge disaster. Unless you already have lots of small services with clean interfaces and their own data stores that can be incrementally switched, you spend far more time bending over backwards to keep two systems running (one with some limited parity with the other) in tandem.

The cost of running the system is almost never bottlenecked by the performance of the language itself, but rather the responsibilities of the system and cleanliness of the code. Plus, in the migration, you start incurring the cost of your Ruby code calling your JVM code (and vise versa; your systems are almost certainly not a DAG), which almost certainly has higher overhead than whatever speedup you'd get from running code in the JVM in the first place. And then you're sharing protobufs/thrift files/whatever between different languages and libraries of varying degrees of quality (good luck with those Ruby protobufs!).

Before you know it, writing a little tool to optimize your Ruby garbage collector sounds like a really great idea.