Rails server is mostly for development mode. When deployed I think most people use either Unicorn, or throw their applications on JRuby that runs on the JVM with some kind of appserver.

JRuby have this advantage of being multithreaded, so you can parallelize within a single process, and don't rely on forking. Stock Ruby with MRI have a GIL, and as far as I know only runs on one core.

The limitations of stock Ruby is being worked on, but there is still a long way.