A hack to measure how long it takes to require bundled gems in a rails app

require_measure.rb

REQUIRE_DATA = {}
BUNDLED_GEMS = `bundle show | grep "*" | awk '{print $2}'`.split("\n").sort
require 'bigdecimal'

def require(name)
  start = Time.now
  ret = super
  stop = Time.now
  total = BigDecimal(stop-start, 6)
  REQUIRE_DATA[name] = total if BUNDLED_GEMS.include?(name)
  ret
end

def require_summary!
  require_info = REQUIRE_DATA.to_a.sort_by(&:last).reverse

  File.open("requires.log", "w") do |f|
    require_info.each do |gem, time|
      f << "#{gem}|#{time}\n"
    end
  end
end

requires.log

sass-rails|0.350504
rails-html-sanitizer|0.0841502
loofah|0.0807085
nokogiri|0.06919
slim|0.0590941
coffee-rails|0.0531291
coffee-script|0.0510261
execjs|0.0493581
arel|0.042798
validates_timeliness|0.0417477
timeliness|0.0228528
tzinfo|0.0221143
gretel|0.020622
concord|0.0199939
adamantium|0.0181447
axiom-types|0.0180199
erubis|0.0161383
coercible|0.0148004
rom-sql|0.0141706
rom-rails|0.0109552
warden|0.0062394
bcrypt|0.00602871
memoizable|0.00388941
i18n|0.00387294
inflecto|0.00230061
bootstrap-sass|0.00190944
jquery-rails|0.00149442
transproc|0.00140992
jquery-ui-rails|0.00140623
charlatan|0.0012355
temple|0.000799056
ref|0.000597096
hike|0.000403201
rspec-rails|0.000357722
rails|0.000013796
tilt|0.000012404
thread_safe|0.00001217
json|0.00001214
thor|0.000011942
neat|0.000011416
virtus|0.000011168
rom|0.000010758
sequel|0.000010697
sass|0.00001034
sprockets|0.000009684
descendants_tracker|0.00000799
bourbon|0.000007212
equalizer|0.000006448
bitters|0.000006435
ice_nine|0.000006426
refills|0.000005494
rack|0.00000447

usage.rb

# paste above hack at the top of your config/application.rb

# HERE GOES YOUR application.rb

# at the end:
require_summary!

# look at requires.log that was created

As pointed out - it would give much better results when you run it in an isolated script where you require individual gem once and append time to the log.