Compilers / runtimes for most OOP languages don't really put the same level of optimization into operations with immutable structures as they do with mutable on

Compilers / runtimes for most OOP languages don't really put the same level of optimization into operations with immutable structures as they do with mutable ones, because the mutable paradigm is largely how the languages are (intended to be) used. There are exceptions, of course, but you are leaving a lot on the table when you try to shoehorn one into the other.