It's certainly not "free", but as the method_missing calls are all outside of the loop ((0...1000).map(&X.to_s) calls method_missing only once) the impact on code is minimal.

I've also found using perftools.rb that it's pretty easy to spot ampex — there was one place where we were using it in a tight nested loop and removing that made a measurable difference over tens of thousands of iterations (just as converting &block parameters to implicit yields makes a measurable difference); but mostly the causes of slowness in our app have not been this.