My friend Brent (site) has been experimenting with some ruby code optimizations for joining strings. It appears you can squeeze quite a bit of more performance with a small change in code.

Here is the original code which took 13.8 seconds:

a = ''
(0..10000).each do |counter|
  a += 'a'
end

Now an alternative version that is a bit faster (0.127 seconds):

a = ''
(0..10000).each do |counter|
   a << 'a'
end

So if you want to join a lot of strings you may want to consider the alternate method. It may save you some time.