The interpreter in 1.8 and earlier is definitely pretty sluggish; I believe the 1.9 implementation was intended to be quite a lot faster, and JRuby is pretty fast. No idea about Rubinius or IronRuby or other alternative implementations.

It's definitely the case that stock Ruby 1.8 is slow for a great many applications, though. Having said that, there's alternatives, and a lot of the time efficiency of execution is not at the forefront of the objectives for systems written in Ruby.