https://medium.freecodecamp.org/how-to-optimize-react-applications-with-lazy-loading-232183e02768

Lazy loading is an old technique to optimize web applications as well as on mobile apps. The thing is pretty straight forward - do not render things if they are not viewed or required at that moment. So, for example, if we have a list of posts to show, we should initially only render what’s on the viewport. That means the rest of the elements will be rendered later on demand (when they’re in the viewport or about to be on the viewport).

Most of the times our users don’t see the whole web page, at least in the beginning. No matter how our application UI has been structured, there are certain components the user might not need initially or ever!