This is in no way a good test. Yes, Rubinius takes longer to load than MRI. It'a also JITed, so it needs time to warm up to start screaming.

Also, this is in dev mode... did you disable Rack::Lock?

MRI can only handle 1 request at a time, Rubinius can handle whatever your concurrency level is (assuming you've removed Rack::Lock from your environment)

Rails 4 is thread safe and I used puma as server Also the app is in production mode and the db pool is 16