Why is faster : ENV["key"] or ENV.key?("key") ? The answer will surprise you ...

ruby-benchmark-env.key-or-env.bracket.rb

require "benchmark/ips"

HASH = ("a".."zz").to_a.shuffle.to_h { |e| [e, "#{e}#{e}#{e}"] }
KEY = "zz"

def key_fast
  HASH.key? KEY
end

def key_slow
  HASH[KEY]
end

Benchmark.ips do |x|
  x.report("Hash#[]") { key_slow }
  x.report("Hash#key?") { key_fast }
  x.compare!
end