Until Ruby 3.2 the `is_a?` method can be a surprising performance bottleneck. It be called directly or through its various synonyms like case statements, rescue statements, protected methods, `Module#===` and more! Recently `is_a?` and its various flavours have been optimized and it's now faster and runs in constant time. Join me in the journey of identifying it as a bottleneck in production, implementing the optimization, squashing bugs, and finally turning it into assembly language in YJIT.