test.rb

require 'benchmark'
p ["Encodings: ", Encoding.default_external, Encoding.default_internal]

strings = {
  10 => " " * 10,
  100 => " " * 100,
  1000 => " " * 1000,
  10_000 => " " * 10_000,
  100_000 => " " * 100_000,
  1_000_000 => " " * 1_000_000
}

reps = 1000

Benchmark.bm do |x|
  strings.each do |length, string|
    x.report("Length: #{length}") do
      reps.times do
        (-10..-1).each do |i|
          string[i] = "x"
        end
      end
    end
  end
end