Most of the time, React is fast enough by default and you do not need to do anything more to improve the performance of your application. React utilizes different techniques to optimize the rendering of the components on the screen.
When React has to display a component, it calls its render method and the render methods of its children recursively. The render method of a component returns a tree of React elements, which React uses to decide which DOM operations have to be done to update the UI.
Whenever a component's state changes, React calls the render methods on the nodes again and it compares the result with the previous tree of React elements. The library is smart enough to figure out the minimum set of operations required to apply the expected changes on the screen. This process is called reconciliation and it is managed transparently by React. Thanks to that, we can easily describe how our components have to look at a given point in time in a declarative way...