JSON under IronRuby (and comparisons)

ironruby.txt

C:\IronRuby\v1.1.0>ir json_speed.rb pure
Rehearsal ------------------------------------
 474.906250  39.421875 514.328125 (394.262696)
------------------------- total: 514.328125sec

       user     system      total        real
 482.531250  43.140625 525.671875 (384.509766)

C:\IronRuby\v1.1.0>ir json_speed.rb ext
Rehearsal ------------------------------------
   0.140625   0.000000   0.140625 (  0.145508)
--------------------------- total: 0.140625sec

       user     system      total        real
   0.062500   0.000000   0.062500 (  0.071289)

jruby.txt

C:\IronRuby\v1.1.0>jruby json_speed.rb pure
Rehearsal ------------------------------------
   1.649000   0.000000   1.649000 (  1.649000)
--------------------------- total: 1.649000sec

       user     system      total        real
   0.673000   0.000000   0.673000 (  0.673000)

C:\IronRuby\v1.1.0>jruby json_speed.rb ext
Rehearsal ------------------------------------
   0.303000   0.000000   0.303000 (  0.302000)
--------------------------- total: 0.303000sec

       user     system      total        real
   0.096000   0.000000   0.096000 (  0.096000)

json_speed.rb

require 'open-uri'
require 'benchmark'

variant = ARGV[0] || 'ext'
require "json/#{variant}"

data = open('510kbyte.json').read

Benchmark.bmbm do |x|
    x.report do
        JSON.parse(data)
    end
end

ruby.txt

C:\IronRuby\v1.1.0>ruby json_speed.rb pure
Rehearsal ------------------------------------
   0.625000   0.000000   0.625000 (  0.624000)
--------------------------- total: 0.625000sec

       user     system      total        real
   0.610000   0.000000   0.610000 (  0.615000)

C:\IronRuby\v1.1.0>ruby json_speed.rb ext
Rehearsal ------------------------------------
   0.047000   0.000000   0.047000 (  0.036000)
--------------------------- total: 0.047000sec

       user     system      total        real
   0.031000   0.000000   0.031000 (  0.023000)

I wonder why json/pure performs that bad under IronRuby, even after the first run (which is even more surprising since it's actually slower in the second run). I can't really figure where's the bottleneck...