Functional programming builds immutable semantics as an abstraction on top of mutable semantics. This abstraction layer includes internal optimizations that elide unnecessary copying, such as persistent data structures and "list fusion." When you're processing a list, the compiler can optimize away intermediate sublists. If you "copy" a list or sublist in Haskell, you don't actually get a copy, you get a new pointer to the same physical data, which is safe because it's immutable and garbage collected.

A key premise of functional programming is that the constraints of immutable semantics free the compiler up to do all sorts of performance optimizations, and the compiler can apply those optimizations much faster than a human developer can.

Also, when you really need a vector that you can mutate in place, you can still do that. Haskell has a mutable vector type. It's not the default idiom, but if you needed to sort a billion numbers in place, this is what you would use. Note all the function names prefixed with "unsafe": https://hackage.haskell.org/package/vector-0.12.1.2/docs/Dat...