RE: http://eval.in/4817 ... Singleton methods add some cost of execution to objects in MRI 1.9.3, not much in MRI 1.8.7, and JRuby doesn't seem to be affected. Note: the original anonymous Gist was mine also (didn't realize I wasn't logged in).

output

~/.rvm/rubies/jruby-1.6.5/bin/ruby
before singleton method definition
  0.991000   0.000000   0.991000 (  0.990000)
after singleton method definition
  0.641000   0.000000   0.641000 (  0.641000)


~/.rvm/rubies/jruby-1.6.7.2/bin/ruby
before singleton method definition
  0.554000   0.000000   0.554000 (  0.555000)
after singleton method definition
  0.502000   0.000000   0.502000 (  0.502000)


~/.rvm/rubies/jruby-1.6.8/bin/ruby
before singleton method definition
  0.530000   0.000000   0.530000 (  0.530000)
after singleton method definition
  0.510000   0.000000   0.510000 (  0.510000)


~/.rvm/rubies/jruby-1.7.0/bin/ruby
before singleton method definition
  1.730000   0.040000   1.770000 (  0.859000)
after singleton method definition
  0.700000   0.000000   0.700000 (  0.518000)


~/.rvm/rubies/jruby-1.7.1/bin/ruby
before singleton method definition
  1.610000   0.040000   1.650000 (  0.779000)
after singleton method definition
  0.760000   0.000000   0.760000 (  0.578000)


~/.rvm/rubies/jruby-head/bin/ruby
before singleton method definition
  1.540000   0.050000   1.590000 (  0.775000)
after singleton method definition
  0.710000   0.010000   0.720000 (  0.526000)


~/.rvm/rubies/ree-1.8.7-2012.02/bin/ruby
before singleton method definition
  0.830000   0.000000   0.830000 (  0.828132)
after singleton method definition
  0.800000   0.000000   0.800000 (  0.806208)


~/.rvm/rubies/ruby-1.8.7-p352/bin/ruby
before singleton method definition
  0.930000   0.000000   0.930000 (  0.930660)
after singleton method definition
  0.940000   0.000000   0.940000 (  0.942419)


~/.rvm/rubies/ruby-1.8.7-p358/bin/ruby
before singleton method definition
  0.960000   0.000000   0.960000 (  0.962650)
after singleton method definition
  0.950000   0.000000   0.950000 (  0.954485)


~/.rvm/rubies/ruby-1.8.7-p370/bin/ruby
before singleton method definition
  0.960000   0.000000   0.960000 (  0.963616)
after singleton method definition
  0.950000   0.000000   0.950000 (  0.947487)


~/.rvm/rubies/ruby-1.9.3-p194/bin/ruby
before singleton method definition
  0.160000   0.000000   0.160000 (  0.166036)
after singleton method definition
  0.410000   0.000000   0.410000 (  0.414524)


~/.rvm/rubies/ruby-1.9.3-p286/bin/ruby
before singleton method definition
  0.390000   0.000000   0.390000 (  0.392831)
after singleton method definition
  1.050000   0.000000   1.050000 (  1.050109)


~/.rvm/rubies/ruby-1.9.3-p327/bin/ruby
before singleton method definition
  0.180000   0.000000   0.180000 (  0.185082)
after singleton method definition
  0.410000   0.000000   0.410000 (  0.401608)

singleton_method_measurement.rb

require "benchmark"
include Benchmark

puts "\n\n#{`which ruby`.chomp}"

GC.disable unless RUBY_PLATFORM =~ /java/

upper_boundary = ARGV[0].to_i || 100

TIMES = 1_000

eval "def run; 10_000.times { #{"$a[5]\n" * TIMES} } end"

$a = (1..upper_boundary).to_a

puts "before singleton method definition #{$a.size} elements"
puts measure { run }

def $a.x; end

puts "after singleton method definition #{$a.size} elements"
puts measure { run }