benchmark

comp.rb

require 'benchmark'

n = 1000000
words = ["string", :symbol, "string", "string", "string"]
Benchmark.bm(20) do |x| 

  x.report('<< "_"') do
    n.times do
      res = words.inject("") {|string, p| string << "#{p}_"}
    end
  end

  x.report("join('_')") do 
    n.times do
      res = words.map{|s| s}.join("_")
    end
  end
  
  x.report("join()") do 
    n.times do
      res = words.map{|s| "#{s}_"}.join
    end
  end

  x.report("tained") do
    n.times do
      res = words.map {|s| s.tainted? }
    end
  end

  x.report("loop") do
    n.times do
      res = words.map {|s| s}
    end
  end

end

results

(ruby-1.9.2-p136) 
⚡ ruby comp.rb 
                          user     system      total        real
<< "_"                4.260000   0.020000   4.280000 (  4.475740)
join('_')             2.690000   0.000000   2.690000 (  2.694680)
join()                4.060000   0.010000   4.070000 (  4.070008)
tained                1.300000   0.000000   1.300000 (  1.301931)
loop                  1.020000   0.000000   1.020000 (  1.023793)

(rbx-head) 
⚡ ruby comp.rb 
                          user     system      total        real
<< "_"                5.090899   0.012431   5.103330 (  5.069069)
join('_')             5.184881   0.008267   5.193148 (  5.077835)
join()                5.289410   0.009593   5.299003 (  5.292927)
tained                1.322216   0.007856   1.330072 (  1.322064)
loop                  1.134619   0.001723   1.136342 (  1.125007)

(1.8.7-p174) 
⚡ ruby comp.rb 
                          user     system      total        real
<< "_"                9.480000   0.010000   9.490000 (  9.528440)
join('_')             3.070000   0.000000   3.070000 (  3.097907)
join()                5.480000   0.010000   5.490000 (  5.488951)
tained                1.980000   0.000000   1.980000 (  1.982910)
loop                  1.600000   0.000000   1.600000 (  1.608543)