fib.rb

def fib(n)
  return n if n < 2
  fib(n-1) + fib(n-2)
end

Benchmark.bm do |x|
  x.report { fib 40 }
end

gistfile1.txt

gauss:rubinius brian$ ruby -r benchmark fib.rb 
      user     system      total        real
153.620000   0.360000 153.980000 (157.128205)
gauss:rubinius brian$ ruby1.9 -r benchmark fib.rb 
      user     system      total        real
 33.450000   0.100000  33.550000 ( 34.083818)
gauss:rubinius brian$ bin/rbx -r benchmark fib.rb 
      user     system      total        real
 27.729074   0.000000  27.729074 ( 27.729094)
gauss:rubinius brian$ bin/rbx -XJ -r benchmark fib.rb 
      user     system      total        real
  9.568518   0.000000   9.568518 (  9.568531)