Immutability is often promoted to work around the complexity introduced by state management patterns in modern JS. If your state is isolated and you don't need features like time travel debugging, mutable data structures can be simpler and faster. Some so-called immutable libraries use hidden mutations or copy-on-write, which can actually make things slower or harder to reason about. Unless you have a specific need for immutability, starting with mutable structures is usually more sane.