The current (and quite likely the future) star of the Scala web frameworks is Lift, although you can use any other Java framework like Play with Scala, too.
You don't have to fear any performance hit when moving from Ruby to Scala/Lift, expect it to run faster (I heard numbers between 600% and 2000% faster than Ruby on Rails), but it depends on what you are doing.
Here are two short explanations from the creator of Lift about what Lift does and why it might be interesting for people coming from Rails.
