Twitter did not think Rails was particularly adequate at serving up anything (for their extreme needs), so they dumped it for the JVM.

Charles Nutter is doing genius work with JRuby, btw, that's where Ruby has a future with the enterprise, on the JVM. Twitter dumping Rails for Scala was a major win for Odersky & friends, and a big hit to Rails (though Ruby/Rails continues to innovate, nothing has changed there)

Not sure why client-side applications take a second or 2 to render a page, but if that is the case, cached html on front end server would be dee way 2 go, just avoid hitting the application server entirely...