This test https://github.com/fastruby/fast-ruby/blob/master/code/general/block-apply-method.rb is comparing:

def fast
  [1, 2, 3].map { |n| do_something(n) }
end

def slow
  [1, 2, 3].map(&method(:do_something))
end

We could have one more test:

def other
  [1, 2, 3].map(&:do_something)
end

Also, I think the names fast and slow shouldn't be used for the methods, the names should describe what the method does, not which one is faster?