bytes.rb

# encoding: utf-8
require "securerandom"
require "benchmark"

COUNT = 10000000
STRINGS = COUNT.times.map{|i| SecureRandom.hex}

Benchmark.bmbm(30) do |b|
  b.report("force_encoding") do
    tmp = ""
    STRINGS.each do |s|
      tmp = s.dup.force_encoding(Encoding::ASCII_8BIT)
    end
  end

  b.report("byteslice") do
    tmp = ""
    STRINGS.each do |s|
      tmp = s.dup.byteslice(0, s.bytesize)
    end
  end
end

result.txt

ruby -J-Xmx8g tmp/perf/bytes.rb
Rehearsal ------------------------------------------------------------------
force_encoding                  23.210000   0.550000  23.760000 (  8.623000)
byteslice                        1.380000   0.000000   1.380000 (  1.153000)
-------------------------------------------------------- total: 25.140000sec

                                     user     system      total        real
force_encoding                   1.410000   0.010000   1.420000 (  1.322000)
byteslice                        1.230000   0.000000   1.230000 (  1.107000)