require 'benchmark'

ITERATIONS = 10_000

Benchmark.bm do |bm|
  bm.report("#<< -> #flatten!: ") do
    array = [1,2,3]
    ITERATIONS.times do
      array << [4,5,6]
      array.flatten!
    end
  end

  bm.report("+=:               ") do
    array = [1,2,3]
    ITERATIONS.times do
      array += [4,5,6]
    end
  end

  bm.report("#concat:          ") do
    array = [1,2,3]
    ITERATIONS.times do
      array.concat [4,5,6]
    end
  end
end

# OUTCOME
#
# =>        user     system      total        real
# => #<< -> #flatten!:   8.280000   0.200000   8.480000 (  8.473539)
# => +=:                 0.240000   0.350000   0.590000 (  0.599704)
# => #concat:            0.000000   0.000000   0.000000 (  0.001990)