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.
The problem is the "just switching" typically means a full rewrite and has a ton of logistic challenges. Are you gonna higher an entire second team? What happens to feature work on the current system? If you keep going full-throttle on features, you'll never catch up. If you stop developing features for a prolonged time, you are putting your entire business at risk.
Writing new services in Java might help, but still doesn't solve the main issue if your Rails app is a monolith. Breaking up the monolith might be a bad decision and is also super expensive and might even eat lots of the potential performance gains.
JRuby is far behind CRuby, so there are a lot of gems that you can't use, also there are not too many companies investing in JRuby itself, IMHO CRuby it's a safer bet.
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.