gistfile1.rb

require 'benchmark'

def where(app)
  ENV['PATH'].split(File::PATH_SEPARATOR).each do |path|
    return File.join(path, app) if File.exist?(File.join(path, app)) && File.executable?(File.join(path, app))
  end
end

n = 1_000

Benchmark.bmbm do |b|
  b.report('system call') { for i in 1..n; `which ruby`; end }
  b.report('pure-ruby') { for i in 1..n; where('ruby'); end }
end

gistfile2.txt

Rehearsal -----------------------------------------------
system call   0.040000   0.090000   1.170000 (  1.170017)
pure-ruby     0.000000   0.020000   0.020000 (  0.020001)
-------------------------------------- total: 1.190000sec

                  user     system      total        real
system call   0.000000   0.090000   0.970000 (  0.970014)
pure-ruby     0.020000   0.010000   0.030000 (  0.030001)