README.md

$ docker run --rm -v $(pwd):/app -w /app ruby:2.3.7 bash -c "gem install benchmark-ips; ruby /app/benchmark.rb"
Successfully installed benchmark-ips-2.7.2
1 gem installed
{:a=>"1", :b=>2, :c=>nil}
Warming up --------------------------------------
`Hash#[:a] || 3` (key exist)
                       148.791k i/100ms
`Hash#[:c] || 3` (key is nil)
                       141.521k i/100ms
`Hash#[:d] || 3` (key does not exist)
                       136.853k i/100ms
`Hash.fetch(:a) { 3 }` (key exist)
                       136.026k i/100ms
`Hash.fetch(:c) { 3 }` (key is nil)
                       133.084k i/100ms
`Hash.fetch(:d) { 3 }` (key does not exist)
                       121.019k i/100ms
`Hash.fetch(:a, 3)` (key exist)
                       126.041k i/100ms
`Hash.fetch(:c, 3)` (key is nil)
                       126.434k i/100ms
`Hash.fetch(:d, 3)` (key does not exist)
                       117.635k i/100ms
Calculating -------------------------------------
`Hash#[:a] || 3` (key exist)
                          5.080M (± 2.6%) i/s -     25.443M in   5.011909s
`Hash#[:c] || 3` (key is nil)
                          4.086M (± 3.6%) i/s -     20.521M in   5.029719s
`Hash#[:d] || 3` (key does not exist)
                          4.020M (± 1.4%) i/s -     20.117M in   5.005328s
`Hash.fetch(:a) { 3 }` (key exist)
                          4.928M (± 1.3%) i/s -     24.757M in   5.024695s
`Hash.fetch(:c) { 3 }` (key is nil)
                          4.919M (± 1.4%) i/s -     24.621M in   5.005663s
`Hash.fetch(:d) { 3 }` (key does not exist)
                          3.471M (± 1.4%) i/s -     17.427M in   5.021451s
`Hash.fetch(:a, 3)` (key exist)
                          4.003M (± 1.8%) i/s -     20.041M in   5.007797s
`Hash.fetch(:c, 3)` (key is nil)
                          4.170M (± 8.2%) i/s -     20.735M in   5.006985s
`Hash.fetch(:d, 3)` (key does not exist)
                          4.557M (± 5.3%) i/s -     22.821M in   5.023383s

Comparison:
`Hash#[:a] || 3` (key exist):  5080160.7 i/s
`Hash.fetch(:a) { 3 }` (key exist):  4927876.7 i/s - same-ish: difference falls within error
`Hash.fetch(:c) { 3 }` (key is nil):  4919496.8 i/s - same-ish: difference falls within error
`Hash.fetch(:d, 3)` (key does not exist):  4557139.1 i/s - 1.11x  slower
`Hash.fetch(:c, 3)` (key is nil):  4169658.5 i/s - 1.22x  slower
`Hash#[:c] || 3` (key is nil):  4085957.3 i/s - 1.24x  slower
`Hash#[:d] || 3` (key does not exist):  4020012.1 i/s - 1.26x  slower
`Hash.fetch(:a, 3)` (key exist):  4003104.0 i/s - 1.27x  slower
`Hash.fetch(:d) { 3 }` (key does not exist):  3471148.5 i/s - 1.46x  slower

benchmark.rb

require "benchmark/ips"

HASH = { a: "1", b: 2, c: nil }

puts HASH.inspect

Benchmark.ips do |x|
  x.report("`Hash#[:a] || 3` (key exist)") { HASH[:a] || 3 }
  x.report("`Hash#[:c] || 3` (key is nil)") { HASH[:c] || 3 }
  x.report("`Hash#[:d] || 3` (key does not exist)") { HASH[:d] || 3 }

  x.report("`Hash.fetch(:a) { 3 }` (key exist)") { HASH.fetch(:a) { 3 } }
  x.report("`Hash.fetch(:c) { 3 }` (key is nil)") { HASH.fetch(:c) { 3 } }
  x.report("`Hash.fetch(:d) { 3 }` (key does not exist)") { HASH.fetch(:d) { 3 } }

  x.report("`Hash.fetch(:a, 3)` (key exist)") { HASH.fetch(:a, 3) }
  x.report("`Hash.fetch(:c, 3)` (key is nil)") { HASH.fetch(:c, 3) }
  x.report("`Hash.fetch(:d, 3)` (key does not exist)") { HASH.fetch(:d, 3) }

  x.compare!
end