The complete question and answer is here:
My point is:
Let me show to you all my experience with that.
I had an query that returned 32k of records, for each record I called a method to format that database record into a formated string and than concatenate that into a String that at the end of all this process wil turn into a file in disk.
My problem was that by the record goes, around 24k, the process of concatenating the String turned on a pain.
I was doing that using the regular '+' operator.
When I changed to the '<<' was like magic. Was really fast.
So, I remembered my old times - sort of 1998 - when I was using Java and concatenating String using '+' and changed from String to StringBuffer (and now we, Java developer have the StringBuilder).
I believe that the process of + / << in Ruby world is the same as + / StringBuilder.append in the Java world.
The first reallocate the entire object in memory and the other just point to a new address.