flat_map はやい！

benchmark_map_and_flatten.rb

require 'benchmark'

array = (1..10_000).map { |n| [n] }

Benchmark.bm do |x|
  x.report('map & flatten:') { array.map { |a| a << 'yuno' }.flatten }
  x.report('map & flatten!:') { array.map { |a| a << 'yuno' }.flatten! }
  x.report('flat_map:') { array.flat_map { |a| a << 'yuno' } }
end

#                     user     system      total        real
# map & flatten:  0.010000   0.000000   0.010000 (  0.008364)
# map & flatten!: 0.000000   0.000000   0.000000 (  0.007843)
# flat_map:       0.010000   0.010000   0.020000 (  0.002672)