The issue is not so much about whether React components themselves are rendered on the client or server, it's about the network: all the JS and other data that

The issue is not so much about whether React components themselves are rendered on the client or server, it's about the network: all the JS and other data that has to be loaded in general before the browser can begin rendering. With React you can use the same component code used on the client side to generate HTML on the server side and send that down the wire for first page load. The browser doesn't have to sit there waiting for entire libraries to load and then start building <div>s.