As you said yourself, only children components will be re-rendered. This is relative to the component that had it's state changed (using setState or the useState hook). So not the whole tree rerenders, only the affected component and it's children. And again, when we speak as "re-render", the render() functions of the components are called and then the VDOM diffing happens. That is, even if child components got -rerendered (in react terms), there might not be a DOM update at all (which is the slow, costly operation).
> That is, even if child components got -rerendered (in react terms), there might not be a DOM update at all (which is the slow, costly operation).
Crawling a large React tree's render stack is also a costly operation. Not as much as DOM manipulation but it is still costly, and React is created in such a way that it requires you to include everything in that stack, even if it never changes. Which is one reason we have Portals.
As you said yourself, only children components will be re-rendered. This is relative to the component that had it's state changed (using setState or the useState hook). So not the whole tree rerenders, only the affected component and it's children. And again, when we speak as "re-render", the render() functions of the components are called and then the VDOM diffing happens. That is, even if child components got -rerendered (in react terms), there might not be a DOM update at all (which is the slow, costly operation).