The new SSR foundation allows us to combine streaming server-side rendering with a new approach to hydration - Selective Hydration! It also prioritizes hydrating the parts the user is interacting with, creating an illusion of instant hydration.
The most significant update of React 18 comes with concurrent rendering. However, with the external store, there are some issues(tearing) that come up with concurrent rendering. To fix these issues, React 18 provides a new hook, useSyncExternalStore.
In complex applications, client apps have to store and manage a large amount of nested data, and if the data is not normalized, it can be incredibly time-consuming for a program to lookup nested data which could become a performance concern.