Well sure but the OP wrote

>if you had to optimize for raw speed, why not choose mutable data?

So in context we are talking about a case where we have to optimize for raw speed.

It doesn’t matter that immutable data is easier to reason about if you don’t have the performance budget to go that route.

Unless you encode ownership into the type system, and then you kind of have the best of both worlds: you don't have functions mutating things unexpectedly or by accident, but you can explicitly opt into mutation when it would be beneficial. To opt into mutation requires you to have exclusive control over data (i.e. nowhere else in your program will mutate this code at the same time), which avoids issues where different threads are trying to change the same data at the same time.