Except you dont need a JVM to get performances in python. Using a JVM destroys any perspective of cheap and easy deveployement. If i have to use a JVM , i'd certainly not use JRuby when i can get scala.
Python's performance is only slightly ahead of Ruby's. Both Python and Ruby are a bad match for CPU intensive tasks(provided we are talking pure python/ruby and not native extensions). Network and IO in general is salvaged by using evented io(or some high level evented framework).
I am not aware of comparable features in Ruby.
Cython is not intrinsic to Python either. I see there is a ruby2c gem, but I have no idea how it compares to Cython.
As for pypy, that's about switching to a different runtime, the same way Ruby people would go to JRuby.
None of this turns Ruby into apples and Python into oranges, I'm afraid.
I doubt that. The fact that jruby is close to bug-by-bug compatibility means that for most projects you can go and develop on mri or rubinius for fast development and use jenkins/travis/... to run your testsuite against jruby. You can then do integration tests as well as staging on jruby to catch anything your testsuite didn't catch. I've seen more than one large project take that course of action and do just fine.
Deployment on jruby isn't any harder than deployment on any other ruby. It's still "puma start" or whichever server you chose. You can however package your app in a war use a standard servlet container such as tomcat or jetty to deploy, but that's strictly optional.
The biggest win for me is dependency management. One my maven file is setup right, I only need a JRE on the host.. Dependency management is a bit trickier with Python, perhaps for Ruby as well.