bench.rb

require 'ostruct'
require 'money'

class Array
  def sum
    inject{|a,b| a+b }
  end
end

TIMES = 1_000
SIZES = [10, 100, 1_000, 10_000]

require 'benchmark'

Benchmark.bmbm do |x|
  SIZES.each do |size|
    ary = ([1] * size).map{ OpenStruct.new(:balance => Money.new(rand(1000))) }
    x.report(".map.sum (#{size})") { TIMES.times { ary.map(&:balance).sum.to_money }}
    x.report(".inject  (#{size})") { TIMES.times { ary.inject(Money.new(0)){|memo, customer| memo + customer.balance } }}
  end
end

report.txt

Rehearsal ---------------------------------------------------
.map.sum (10)     0.030000   0.000000   0.030000 (  0.036481)
.inject  (10)     0.070000   0.000000   0.070000 (  0.069186)
.map.sum (100)    0.480000   0.000000   0.480000 (  0.484167)
.inject  (100)    0.490000   0.000000   0.490000 (  0.494730)
.map.sum (1000)   5.450000   0.010000   5.460000 (  5.460881)
.inject  (1000)   5.380000   0.010000   5.390000 (  5.393002)
.map.sum (4000)  23.440000   0.030000  23.470000 ( 23.495543)
.inject  (4000)  22.270000   0.030000  22.300000 ( 22.324168)
----------------------------------------- total: 57.690000sec

                      user     system      total        real
.map.sum (10)     0.040000   0.000000   0.040000 (  0.036330)
.inject  (10)     0.040000   0.000000   0.040000 (  0.038244)
.map.sum (100)    0.470000   0.000000   0.470000 (  0.466925)
.inject  (100)    0.480000   0.000000   0.480000 (  0.482890)
.map.sum (1000)   5.270000   0.010000   5.280000 (  5.288732)
.inject  (1000)   5.380000   0.010000   5.390000 (  5.393124)
.map.sum (4000)  23.390000   0.030000  23.420000 ( 23.446630)
.inject  (4000)  22.410000   0.030000  22.440000 ( 22.456908)