俺、今日からIO#readlines使うよ!

#!/usr/bin/ruby -wKU
require 'benchmark'
dir = "/path/to/data"
Benchmark.bmbm do |job|
  job.report('IO.foreach') do
    File.foreach(File.join(dir, 'data.txt')) do  |line|
      uid, rid = line.chomp.split(/:/)
    end
  end
  job.report('IO.readlines') do
    File.readlines(File.join(dir, 'data.txt')).each do  |line|
      uid, rid = line.chomp.split(/:/)
    end
  end
  job.report('IO#each') do
    File.open(File.join(dir, 'data.txt'), 'rb') do |f|
      f.each do |line|
        uid, rid = line.chomp.split(/:/)
      end
    end
  end
  job.report('IO#gets') do
    File.open(File.join(dir, 'data.txt'), 'rb') do |f|
      while f.gets
        uid, rid = $_.chomp.split(/:/)
      end
    end
  end
  job.report('IO#readlines') do
    File.open(File.join(dir, 'data.txt'), 'rb') do |f|
      f.readlines.each do |line|
        uid, rid = line.chomp.split(/:/)
      end
    end
  end
end
# >> Rehearsal ------------------------------------------------
# >> IO.foreach     1.740000   1.270000   3.010000 (  3.608147)
# >> IO.readlines   2.390000   0.920000   3.310000 (  3.825693)
# >> IO#each        1.680000   1.300000   2.980000 (  3.559145)
# >> IO#gets        1.080000   1.360000   2.440000 (  2.969118)
# >> IO#readlines   1.460000   0.610000   2.070000 (  2.708037)
# >> -------------------------------------- total: 13.810000sec
# >>
# >>                    user     system      total        real
# >> IO.foreach     1.520000   1.320000   2.840000 (  3.827412)
# >> IO.readlines   1.740000   0.680000   2.420000 (  2.997026)
# >> IO#each        1.460000   1.490000   2.950000 (  3.533643)
# >> IO#gets        1.210000   1.550000   2.760000 (  3.163758)
# >> IO#readlines   1.560000   0.520000   2.080000 (  2.857770)