bench.rb

require 'benchmark'
require 'active_support/core_ext'

class Transaction
  attr_reader :amount

  def initialize(amount)
    @amount = amount
  end
end

@transactions = (1..1_000_000).map { |n| Transaction.new(n) }

puts RUBY_DESCRIPTION

Benchmark.bm(30) do |benchmark|
  benchmark.report("inject with object, +=") do
    @transactions.inject(0) { |total, transaction|
      total += transaction.amount
    }
  end

  benchmark.report("inject with object, +") do
    @transactions.inject(0) { |total, transaction|
      total + transaction.amount
    }
  end

  benchmark.report("map and sum") do
    @transactions.map(&:amount).sum
  end

  benchmark.report("map and reduce with symbol") do
    @transactions.map(&:amount).reduce(:+)
  end

  benchmark.report("map and reduce with proc") do
    @transactions.map(&:amount).reduce(&:+)
  end

  if @transactions.respond_to?(:lazy)
    benchmark.report("lazy map and reduce") do
      @transactions.lazy.map(&:amount).reduce(:+)
    end

    benchmark.report("lazy map and reduce with proc") do
      @transactions.lazy.map(&:amount).reduce(&:+)
    end
  end
end

results.txt

$ ruby bench.rb
ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-darwin13.0]
                                     user     system      total        real
inject with object, +=           0.110000   0.000000   0.110000 (  0.114151)
inject with object, +            0.110000   0.000000   0.110000 (  0.100532)
map and sum                      0.160000   0.010000   0.170000 (  0.164063)
map and reduce with symbol       0.170000   0.000000   0.170000 (  0.183164)
map and reduce with proc         0.190000   0.000000   0.190000 (  0.187234)
lazy map and reduce              0.410000   0.020000   0.430000 (  0.423945)
lazy map and reduce with proc    0.390000   0.000000   0.390000 (  0.387475)