Any examples in particular? Besides name resolution and method dispatch (and callcc, but I think that may have even been removed from Ruby 1.9) none of Ruby's semantics seem all that different from Lua. Am I missing something?
I guess there's also throw/catch and exceptions.
http://blog.headius.com/2009/04/how-jruby-makes-ruby-fast.ht...
Read it, you will find it interesting. The guy also wrote a small language called Duby which used Ruby like syntax but is much more faster, because he didn't include any of those slowing down features.
The Rubyists certainly know about these issues, so you'll have to ask them for details.
Dispatch is worse than you realize due to the hackish optimizations made in MRI: if you monkeypatch a natively-method from Ruby, other native code will call the original by default.