a.txt

Rehearsal -----------------------------------------------------------
normal fib                0.170000   0.000000   0.170000 (  0.173442)
keyword fib               0.000000   0.000000   0.000000 (  0.000036)
keyword fib (optimized)   0.000000   0.000000   0.000000 (  0.000013)
-------------------------------------------------- total: 0.170000sec

                              user     system      total        real
normal fib                0.180000   0.000000   0.180000 (  0.179575)
keyword fib               0.000000   0.000000   0.000000 (  0.000026)
keyword fib (optimized)   0.000000   0.000000   0.000000 (  0.000008)

b.rb

require 'benchmark'

def fib(n)
  n < 2 ? n : fib(n-1) + fib(n-2)
end

def fib_keyword(n, result: n.times.inject([0,1]){|(a,b),*|[b,a+b]}.first)
  result
end

Benchmark.bmbm do |x|
  x.report("normal fib") { fib 30  }
  x.report("keyword fib") { fib_keyword 30 }
  x.report("keyword fib (optimized)") { fib_keyword 30, result: 832040 }
end