ids = (1..1_000_000).to_a; nil
x = rand(1_000_000)

Benchmark.realtime do
  the_set = ids.to_set
  1000.times do
    the_set.include?(x)
  end
end
# => 0.7543990612030029

Benchmark.realtime do
  1000.times do
    ids.include?(x)
  end
end
# => 44.60349249839783