Unfortunately there is a long backlog of PRs to the React docs waiting for review so I don't know if/when this info might be incorporated into the docs. Also React is evolving so fast that it would need to be updated, e.g. to mention memo, and given that fast evolution I can understand why there's a backlog. But I do hope that someone on the React team eventually has time to look at adding more explanation to the "Optimizing Performance" section.
1. We support using mutable objects in props. Even if a prop is reference identical to the last render, the component might be reading some property of it that has changed.
2. Checking equality of props takes time. In the (common) case where a parent rerenders and it’s children need to change in response, any time we spend comparing will be extra overhead on top of the actual render cost. Often rerendering is fast anyway and adding more comparisons only makes it slower.
Because of these, we don’t do these checks by default. Instead, we recommend adding memo/PureComponent/shouldComponentUpdate in places where you know it will make a difference.