hash_merge_benchmark.rb

require 'securerandom'
require 'benchmark/ips'

hashs = 20.times.map do
  20.times.map { [SecureRandom.hex(2), SecureRandom.hex(2)] }.to_h
end

Benchmark.ips do |x|
  x.time = 5
  x.warmup = 2

  x.report("merge") { hashs.reduce(&:merge) }
  x.report("merge!") { hashs.reduce({}, &:merge!) }
  x.compare!
end

# Warming up --------------------------------------
#                merge    52.000  i/100ms
#               merge!   896.000  i/100ms
# Calculating -------------------------------------
#                merge    629.419  (±24.8%) i/s -      2.860k in   5.040327s
#               merge!      9.915k (±29.9%) i/s -     43.904k in   5.069974s

# Comparison:
#               merge!:     9915.1 i/s
#                merge:      629.4 i/s - 15.75x slower