Benchmark Example

require 'benchmark'

n = 100000
Benchmark.bm do |x|
  x.report('copy') { n.times do ; h = {}; h = h.merge({1 => 2}); end }
  x.report('no copy') { n.times do ; h = {}; h.merge!({1 => 2}); end }
end

#          user         system       total        real
# copy     0.460000    0.180000    0.640000 (  0.640692)
# no copy  0.340000    0.120000    0.460000 (  0.463339)