Why is EventMachine's defer slower than a Ruby Thread?

I have two scripts which use Mechanize to fetch a Google index page. I assumed EventMachine will be faster than a Ruby thread, but it's not.

EventMachine code costs: "0.24s user 0.08s system 2% cpu 12.682 total"

Ruby Thread code costs: "0.22s user 0.08s system 5% cpu 5.167 total "

Am I using EventMachine in the wrong way?

EventMachine:

require 'rubygems'
require 'mechanize'
require 'eventmachine'

trap("INT") {EM.stop}

EM.run do 
  num = 0
  operation = proc {
    agent = Mechanize.new
    sleep 1
    agent.get("http://google.com").body.to_s.size
  }
  callback = proc { |result|
    sleep 1
    puts result
    num+=1
    EM.stop if num == 9
  }

  10.times do 
    EventMachine.defer operation, callback
  end
end

Ruby Thread:

require 'rubygems'
require 'mechanize'


threads = []
10.times do 
  threads << Thread.new do 
    agent = Mechanize.new
    sleep 1
    puts agent.get("http://google.com").body.to_s.size
    sleep 1
  end
end


threads.each do |aThread| 
  aThread.join
end