test_merge.rb

Bundler.setup(:default, :benchmarks)
Bundler.require

require 'benchmark'
require 'benchmark/ips'

b = { b: :b }

Benchmark.ips do |x|
  x.report('merge') { { a: :a }.merge(b) }
  x.report('keywords') { { a: :a, **b } }
  x.compare!
end

__END__

Warming up --------------------------------------
               merge    51.889k i/100ms
            keywords   100.272k i/100ms
Calculating -------------------------------------
               merge    584.229k (± 8.7%) i/s -      2.906M in   5.010994s
            keywords      1.686M (± 6.5%) i/s -      8.423M in   5.019280s

Comparison:
            keywords:  1685535.9 i/s
               merge:   584228.6 i/s - 2.89x slower