I found it correct: what the says is that we should focus first on the actual bottlenecks before we worry about other optimizations; not that we should just loo

I found it correct: what the says is that we should focus first on the actual bottlenecks before we worry about other optimizations; not that we should just look to another side. If your bottle neck is in that point (copy of data structures) then there are functional data structures as well. And if still your performance is so critical in that point you can use a mutable data structure which is thread safe, and separate data from code in the rest of your code having still a big chunk of functional code where it is easy to reason about.