Immutable data structures and algorithms don't copy immutable state nearly as much as one might assume, and the vast majority of our field doesn't write software that is affected by the tiny performance differences between the underlying implementations of OOP Java versus functional Clojure, or perfectly optimized OOP JS versus naive functional Elm.
Sometimes its fine and sometimes its not but that is why 100% pure isn't feasible. I personally try take functional ideas into my OOP language as much as possible.
