Multithreaded script performs slower than singlethreaded

multithreaded.rb

require 'benchmark'

puts(Benchmark.measure do
  count = 0
  400_000_000.times do
    count += 1
  end
end)

puts(Benchmark.measure do
  (1..4).map do
    Thread.new do
      count = 0
      100_000_000.times do
        count += 1
      end
    end
  end.each(&:join)
end)

⇒  ruby -v multithreaded.rb
jruby 9.0.4.0 (2.2.2) 2015-11-12 b9fb7aa Java HotSpot(TM) 64-Bit Server VM 25.72-b15 on 1.8.0_72-b15 +jit [darwin-x86_64]
 12.490000   0.120000  12.610000 ( 11.768930)
 64.580000   0.100000  64.680000 ( 17.294624)
⇒  ruby -v multithreaded.rb
ruby 2.2.1p85 (2015-02-26 revision 49769) [x86_64-darwin14]
 19.880000   0.010000  19.890000 ( 19.905881)
 21.120000   0.110000  21.230000 ( 21.348419)