String#<< vs String#+=

test.rb

# More details about the 23 value
# https://github.com/ruby/ruby/blob/e279eca4bdf1e4364308aee2ca504bbdf2b5d221/include/ruby/ruby.h#L824-836
#
require 'benchmark'

N = 1_000_000

Benchmark.bmbm do |bm|
  bm.report("s_23chr +=") { N.times { s_23chr = "xxxxxxxxxxxxxxxxxxxxxx"; s_23chr += "x" } }
  bm.report("s_23chr <<") { N.times { s_23chr = "xxxxxxxxxxxxxxxxxxxxxx"; s_23chr << "x" } }

  bm.report("s_24chr +=") { N.times { s_24chr = "xxxxxxxxxxxxxxxxxxxxxxx"; s_24chr += "x" } }
  bm.report("s_24chr <<") { N.times { s_24chr = "xxxxxxxxxxxxxxxxxxxxxxx"; s_24chr << "x" } }
end

# Results
# =======
#
# ---- MRI ----
# λ ruby test.rb
# Rehearsal ----------------------------------------------
# s_23chr +=   0.530000   0.000000   0.530000 (  0.588668)
# s_23chr <<   0.520000   0.000000   0.520000 (  0.566326)
# s_24chr +=   0.950000   0.010000   0.960000 (  1.166513)
# s_24chr <<   0.940000   0.000000   0.940000 (  1.036913)
# ------------------------------------- total: 2.950000sec
#
# user     system      total        real
# s_23chr +=   0.530000   0.000000   0.530000 (  0.593657)
# s_23chr <<   0.520000   0.000000   0.520000 (  0.625555)
# s_24chr +=   0.950000   0.010000   0.960000 (  1.106309)
# s_24chr <<   0.940000   0.000000   0.940000 (  1.068339)

# ---- Rubinius ---
# Rehearsal ----------------------------------------------
# s_23chr +=   2.974743   0.036307   3.011050 (  2.560110)
# s_23chr <<   0.515138   0.007274   0.522412 (  0.496798)
# s_24chr +=   1.097682   0.015315   1.112997 (  1.003511)
# s_24chr <<   0.431469   0.008770   0.440239 (  0.487722)
# ------------------------------------- total: 5.086698sec
# 
#                  user     system      total        real
# s_23chr +=   0.915017   0.013043   0.928060 (  0.967486)
# s_23chr <<   0.402066   0.005867   0.407933 (  0.445658)
# s_24chr +=   0.732000   0.015172   0.747172 (  0.825339)
# s_24chr <<   0.333328   0.007682   0.341010 (  0.352961)