[fbcocq]

I think the speed increase comes from += creating a new string object every time, whereas << literally concatenates it.

Programming the hunches and guesses way on the HN frontpage makes me sad.

http://ruby-doc.org/core/classes/String.html#M000807

str + other_str => new_str

Concatenation Returns a new String containing other_str concatenated to str.

str << fixnum => str str.concat(fixnum) => str str << obj => str str.concat(obj) => str

Append Concatenates the given object to str. If the object is a Fixnum between 0 and 255, it is converted to a character before concatenation.