Not exactly correct or incorrect. Some projects built as a single page app (SPA), using a fancy framework like React or Angular with little configuration, end up loading all DOM content client side via javascript which has a number of drawbacks on performance and SEO.
It doesn't have to be that way though. If you want to use React, there are tools to help you easily make something like a static site generator (SSG) which will transform your React code down into pre-rendered HTML pages. Two popular ones are Gatsby (https://gatsby.dev/) and Next.js (https://nextjs.org/) which also supports Server Side Rendering (SSR).
So out of the box plain HTML / CSS might be more optimized than a fresh create-react-app repo, but if you prefer to use the toolset provided by React to build your site (or Vue or Angular, etc), there are plenty of options to make that equally performant.
It doesn't have to be that way though. If you want to use React, there are tools to help you easily make something like a static site generator (SSG) which will transform your React code down into pre-rendered HTML pages. Two popular ones are Gatsby (https://gatsby.dev/) and Next.js (https://nextjs.org/) which also supports Server Side Rendering (SSR).
So out of the box plain HTML / CSS might be more optimized than a fresh create-react-app repo, but if you prefer to use the toolset provided by React to build your site (or Vue or Angular, etc), there are plenty of options to make that equally performant.