gistfile1.rb

require 'benchmark'

n = 1000000
b = "b string".freeze

Benchmark.bmbm do |x| 
  x.report("use interpola") { n.times do; a = 'a string'; "#{a} #{b}"; end}
  x.report("concat single") { n.times do; a = 'a string'; a + b; end}
  x.report("use << single") { n.times do; a = 'a string'; a << b; end}
end

gistfile2.txt

Rehearsal -------------------------------------------------
use interpola   0.590000   0.000000   0.590000 (  0.591011)
concat single   0.280000   0.000000   0.280000 (  0.286938)
use << single   0.270000   0.000000   0.270000 (  0.261599)
---------------------------------------- total: 1.140000sec

                    user     system      total        real
use interpola   0.580000   0.000000   0.580000 (  0.585043)
concat single   0.300000   0.000000   0.300000 (  0.294077)
use << single   0.250000   0.000000   0.250000 (  0.252018)