results

$ ./start_with_and_index.rb  
                      user     system      total        real
index fail        0.420000   0.000000   0.420000 (  0.424444)
start_with? fail  0.990000   0.000000   0.990000 (  0.993577)
index pass        0.420000   0.000000   0.420000 (  0.424240)
start_with? pass  1.650000   0.020000   1.670000 (  1.672174)

start_with_and_index.rb

#!/usr/bin/env ruby

require 'benchmark'

times = 1000000
string = "this is a string"

Benchmark.bm(16) do |x|
  x.report("index fail") do
    times.times do
      string.index("this is a stripe")
    end
  end

  x.report("start_with? fail") do
    times.times do
      string.start_with?("this is a stripe")
    end
  end

  ####
  
  x.report("index pass") do
    times.times do
      string.index("this is a string")
    end
  end

  x.report("start_with? pass") do
    times.times do
      string.start_with?("this is a string")
    end
  end

end