Simple benchmark (simple test from Rodrigo Alves : https://gist.github.com/rodrigoalvesvieira/6255267)

simple_benchmark.rb

require 'benchmark'

Benchmark.bm(10) do |x| 
  x.report("Array#join") {
		10000.times do
			["'", ("a".."z").to_a.join("' , '"), "'"].join
		end
	}
	x.report("String#<<") {
		10000.times do
			res = ""
			("a".."z").to_a.each do |l|
			  res << "'#{l}'" << ", "
			end
		end
	}
end

                 user     system      total        real
Array#join   0.100000   0.000000   0.100000 (  0.107406)
String#<<    0.230000   0.000000   0.230000 (  0.229317)

--------------------
testing with 50_000 loops

                 user     system      total        real
Array#join   0.520000   0.000000   0.520000 (  0.517532)
String#<<    1.140000   0.000000   1.140000 (  1.149118)