[matchu]

Some immutable data structures implementations perform optimizations under the hood.

For example, if you have a large game state object with mostly stable fields, but the position value changes frequently, you don't need to perform a full copy of the object when position changes. Instead, you could create a new object that only keeps track of the position field, and delegates all requests for other fields to a more stable underlying game state object.

You could also imagine an environment that, if you're going to throw away the reference to the old game state anyway, then it's safe to mutate the object directly instead of bothering with copies or proxies.

…that said, most implementations—especially if you're not careful—will probably incur performance costs, and, at 60 FPS, those matter. They're just not necessarily as bad as you're imagining :)