results 1.9.3

                                      user       system     total        real
Length: 10, encoding: US-ASCII        0.000000   0.000000   0.000000 (  0.002861)
Length: 100, encoding: US-ASCII       0.010000   0.000000   0.010000 (  0.003694)
Length: 1000, encoding: US-ASCII      0.000000   0.000000   0.000000 (  0.003162)
Length: 10000, encoding: US-ASCII     0.000000   0.000000   0.000000 (  0.003325)
Length: 100000, encoding: US-ASCII    0.010000   0.000000   0.010000 (  0.003654)
Length: 1000000, encoding: US-ASCII   0.000000   0.000000   0.000000 (  0.004246)
Length: 10000000, encoding: US-ASCII  0.010000   0.010000   0.020000 (  0.010525)

results 2.3.0

                                      user       system     total        real
Length: 10, encoding: US-ASCII        0.010000   0.000000   0.010000 (  0.003674)
Length: 100, encoding: US-ASCII       0.000000   0.000000   0.000000 (  0.003118)
Length: 1000, encoding: US-ASCII      0.010000   0.000000   0.010000 (  0.003761)
Length: 10000, encoding: US-ASCII     0.010000   0.000000   0.010000 (  0.011070)
Length: 100000, encoding: US-ASCII    0.060000   0.000000   0.060000 (  0.062892)
Length: 1000000, encoding: US-ASCII   0.710000   0.010000   0.720000 (  0.735236)
Length: 10000000, encoding: US-ASCII  7.660000   0.050000   7.710000 (  7.824110)

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,
  10_000_000 => " " * 10_000_000
}

strings.each {|k,v| strings[k] = v.encode "ASCII" }

reps = 1000

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