hash_merge_benchmark.rb

#!/usr/bin/env ruby

require 'benchmark'

Benchmark.bm do |b|
  n = 1_000_000

  hash1 = {a: 1, b: 2, c: 3}
  hash2 = {x: 1, y: 2, z: 3}

  b.report('Hash#merge') do
    n.times { {a: 1, b: 2, c: 3}.merge(hash2) }
  end

  b.report('keyword args') do
    n.times { {a: 1, b: 2, c: 3, **hash2} }
  end
end

user     system      total        real

Hash#merge 1.540000 0.000000 1.540000 ( 1.550766)
keyword args 0.650000 0.000000 0.650000 ( 0.648758)

the results are much closer with Hash#update:

   user     system      total        real

Hash#merge 1.750000 0.010000 1.760000 ( 1.757309)
Hash#update 0.750000 0.000000 0.750000 ( 0.752853)
keyword args 0.700000 0.010000 0.710000 ( 0.709861)