strings_bench.rb

require 'benchmark'

Benchmark.bmbm do |x|
  x.report "double quotes" do
    10_000_000.times do
      "foo"
    end
  end
  x.report "single quotes" do
    10_000_000.times do
      'foo'
    end
  end
end

xoutput

$ ruby strings_bench.rb 
Rehearsal -------------------------------------------------
double quotes   2.810000   0.010000   2.820000 (  2.809821)
single quotes   2.810000   0.000000   2.810000 (  2.815568)
---------------------------------------- total: 5.630000sec

                    user     system      total        real
double quotes   2.810000   0.000000   2.810000 (  2.808336)
single quotes   2.810000   0.000000   2.810000 (  2.812794)