The low performance parts don’t need to be React. Listing pages, static pages, blogs - these things are increasingly built in React, but the benefits they accrue are extremely narrow. A lot of the optimizations that are cropping up in these corners, things like bundle splitting, server-side rendering, and prerendering, are triangulating, essentially, what we had before the rise of React.
This isn't really about React but more about the over-engineering of large swaths of page-types and sites that used to "just work". I love React and it's been a giant leap forward for creating complex UIs on the web, that doesn't mean it's right for every situation.