hash-mark.rb

require 'benchmark'

hash = {}
(1..5_000_000)
  .map { |x| format '%020d', x }
  .each { |k| hash["header#{k}"] = "value#{k.reverse}" }

Benchmark.bm(50) do |x|
  puts "RUBY: #{RUBY_VERSION}"
  x.report('map w/split join')          { hash.map { |k, v| [k, v].join(':') } }
  x.report('map w/split interpolation') { hash.map { |k, v| "#{k}:#{v}" } }
  x.report('map w/split string cat +')  { hash.map { |k, v| k + ':' + v } }
  x.report('map w/split string cat <<') { hash.map { |k, v| '' << k << ':' << v } }
  x.report('map join')                  { hash.map { |m| m.join(':') } }
  x.report('map interpolation')         { hash.map { |m| "#{m.first}:#{m.last}" } }
  x.report('map string cat +')          { hash.map { |m| m.first + ':' + m.last } }
  x.report('map string cat <<')         { hash.map { |m| '' << m.first << ':' << m.last } }
end

results.txt

$ for rv in 1.9.2 1.9.3 2.0.0 2.1.2; do rbenv shell "$rv" && ruby hash-mark.rb; done ; rbenv shell --unset

                                                        user     system      total        real
RUBY: 1.9.2
map w/split join                                    8.690000   0.380000   9.070000 (  9.257528)
map w/split interpolation                           8.860000   0.190000   9.050000 (  9.102149)
map w/split string cat +                            7.510000   0.240000   7.750000 (  7.810475)
map w/split string cat <<                           8.200000   0.200000   8.400000 (  8.378744)
map join                                            6.820000   0.300000   7.120000 (  7.119914)
map interpolation                                   7.880000   0.200000   8.080000 (  8.081020)
map string cat +                                    6.730000   0.180000   6.910000 (  6.900275)
map string cat <<                                   8.260000   0.270000   8.530000 (  8.508148)
                                                         user     system      total        real
RUBY: 1.9.3
map w/split join                                    10.990000   0.330000  11.320000 ( 11.324523)
map w/split interpolation                            9.880000   0.170000  10.050000 ( 10.053683)
map w/split string cat +                             8.860000   0.150000   9.010000 (  8.998289)
map w/split string cat <<                           11.660000   0.180000  11.840000 ( 11.847003)
map join                                             9.790000   0.220000  10.010000 ( 10.002983)
map interpolation                                    9.790000   0.130000   9.920000 (  9.932695)
map string cat +                                     8.370000   0.090000   8.460000 (  8.445293)
map string cat <<                                   11.420000   0.140000  11.560000 ( 11.569196)
                                                         user     system      total        real
RUBY: 2.0.0
map w/split join                                    12.210000   0.330000  12.540000 ( 12.547461)
map w/split interpolation                            7.800000   0.180000   7.980000 (  8.027406)
map w/split string cat +                             7.200000   0.180000   7.380000 (  7.461249)
map w/split string cat <<                            9.730000   0.140000   9.870000 (  9.865895)
map join                                             8.770000   0.230000   9.000000 (  9.184187)
map interpolation                                    8.170000   0.150000   8.320000 (  8.398066)
map string cat +                                     8.790000   0.080000   8.870000 (  8.876930)
map string cat <<                                   10.480000   0.090000  10.570000 ( 10.580655)
                                                         user     system      total        real
RUBY: 2.1.2
map w/split join                                    16.010000   0.340000  16.350000 ( 16.350871)
map w/split interpolation                            9.770000   0.170000   9.940000 (  9.943556)
map w/split string cat +                            10.760000   0.120000  10.880000 ( 10.879686)
map w/split string cat <<                           12.410000   0.180000  12.590000 ( 12.586148)
map join                                            12.900000   0.260000  13.160000 ( 13.176739)
map interpolation                                    9.870000   0.150000  10.020000 ( 10.015435)
map string cat +                                    11.140000   0.080000  11.220000 ( 11.232823)
map string cat <<                                   12.670000   0.160000  12.830000 ( 12.824191)