Drupal is a CMS built with PHP and MySQL. React is a javascript library for building reactive UIs.
React is part of a new generation of front end development, which allows for interactive applications with smooth experiences.
Static Pages
We are also seeing static sites with lightning fast performance, like the Gatsby docs: https://www.gatsbyjs.com/docs/.
Drupal pages can be static if the user is logged out and viewing a piece of content. If they are served by a CDN, a static page load can be just as fast as any other system, the limit is in network and front end performance.
Asset Bundlers
Modern asset bundlers used for js development tend to do a better job of compartmentalizing style and script dependencies. However, this can be achieved with tools in Drupal as well.
Innovative new bundlers like Bun, Vite, Parcel, are built in low level languages and can efficiently bundle application code.
Components
React is built around components, which makes it really nice to work with. Drupal has components in various forms, which can make it difficult to know what's available and how to use them.
This is an area where Drupal has some options, but there is not yet a standard way of defining and implementing components.
One of the major challenges with Drupal theming is a front end developer needs a lot of back end support to implement components due to the php preprocessing.