If you may have an array of arrays, `flat_map` is probably the way to go

flatten_map_bm.rb

require "benchmark"
require "benchmark/ips"

def arr
  Array.new(1_000, 1)
end

def arr_of_arrs
  Array.new(1_000){ (1..1000).to_a }
end

single = arr
nested = arr_of_arrs

puts "Already Flat Array:"
Benchmark.ips do |r|

  r.report("just flatten") do
    single.flatten
  end

  r.report("just flatten(1)") do
    single.flatten(1)
  end

  r.report("flat map identity") do
    single.flat_map{ |x| x }
  end

end

puts
puts "Array of Arrays:"
Benchmark.ips do |r|

  r.report("just flatten") do
    nested.flatten
  end

  r.report("just flatten(1)") do
    nested.flatten(1)
  end

  r.report("flat map identity") do
    nested.flat_map{ |x| x }
  end

end

output.txt

ruby flatten_map_bm.rb
Already Flat Array:
Calculating -------------------------------------
        just flatten      1390 i/100ms
     just flatten(1)      1452 i/100ms
   flat map identity       525 i/100ms
-------------------------------------------------
        just flatten    15291.6 (±6.0%) i/s -      76450 in   5.018510s
     just flatten(1)    14367.3 (±14.6%) i/s -      71148 in   5.095526s
   flat map identity     5466.3 (±7.6%) i/s -      27300 in   5.029275s

Array of Arrays:
Calculating -------------------------------------
        just flatten         1 i/100ms
     just flatten(1)         1 i/100ms
   flat map identity        13 i/100ms
-------------------------------------------------
        just flatten       13.8 (±7.2%) i/s -         69 in   5.061899s
     just flatten(1)       13.4 (±7.4%) i/s -         67 in   5.061220s
   flat map identity      133.2 (±29.3%) i/s -        624 in   5.146658s