string_length.rb

require "benchmark/ips"

C = "123456789012345678901"
A = "1234567890123456789012"
B = "12345678901234567890123"
D = "123456789012345678901234"

def slow
 B + "x"
end

def fast
  A + "x"
end

def slow_ref
  D + "x"
end

def fast_ref
  C + "x"
end

Benchmark.ips do |x|
  x.report("#{C.length} chars") { fast_ref }
  x.report("#{A.length} chars") { fast }
  x.report("#{B.length} chars") { slow }
  x.report("#{D.length} chars") { slow_ref }
end

With ruby 2.6.3 on mac OS 10.14.4 (which makes sense as the internals for String didn't change much since 1.9.3)

Warming up --------------------------------------
            21 chars   273.676k i/100ms
            22 chars   240.588k i/100ms
            23 chars   175.628k i/100ms
            24 chars   174.513k i/100ms
Calculating -------------------------------------
            21 chars      8.209M (± 7.5%) i/s -     40.778M in   5.015295s
            22 chars      8.366M (± 8.0%) i/s -     41.622M in   5.023808s
            23 chars      3.143M (± 7.3%) i/s -     15.631M in   5.013088s
            24 chars      3.112M (± 6.3%) i/s -     15.532M in   5.017577s

Comparison:
            22 chars:  8365985.5 i/s
            21 chars:  8208626.6 i/s - same-ish: difference falls within error
            23 chars:  3143202.6 i/s - 2.66x  slower
            24 chars:  3112300.3 i/s - 2.69x  slower