This seems like such a vague, insubstantial piece of advice. I've seen it before, but never in a more concrete form. I don't see any "magic" in Ruby. How do I determine if a language is magic? Usually, the best I can figure is that they're either confusing syntax with semantics or they're actually thinking of some library like ActiveRecord rather than Ruby.
Really, for pretty much anything I could conceivably think of to call "magic" in Ruby (e.g. method_missing), there's equivalent "magic" in Python (e.g. __get_attr__). And yet PyPy is pretty darned fast.
I think people are referring to language features that are very powerful and concise but have horrible performance when you get large data sets like this.
Something like the inject method, at least in my limited use of ruby with these kind of Google Code questions, seemed problematic.
Really, for pretty much anything I could conceivably think of to call "magic" in Ruby (e.g. method_missing), there's equivalent "magic" in Python (e.g. __get_attr__). And yet PyPy is pretty darned fast.