gistfile1.rb

require 'benchmark/ips'

ARRAY = [*1..100]

def slow
  ARRAY.select { |x| x.eql?(50) }.last
end

def fast
  ARRAY.reverse.detect { |x| x.eql?(50) }
end

Benchmark.ips do |x|
  x.report('slow') { slow }
  x.report('fast') { fast }
  x.compare!
end

ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-darwin14]
Calculating -------------------------------------
                slow     8.011k i/100ms
                fast    11.829k i/100ms
-------------------------------------------------
                slow     87.054k (± 3.6%) i/s -    440.605k
                fast    135.431k (± 3.4%) i/s -    686.082k

Comparison:
                fast:   135430.6 i/s
                slow:    87053.8 i/s - 1.56x slower