Maybe you can provide some proof where the React team admits to this, because that really isn't the case. While yes, class based components could be complex that someone not experienced with may have a hard time with discerning what is going on, that is a case shared with all frameworks, not just React.

Function-based components are infinitely more readable and easier to work with. I do not argue that the size of a barebones React App is fairly large. However, the only reason this would run poorly is if you have a potato PC. My computer is 6+ yrs old. I have no performance issues at all, even considering the fact that my current project is over 900MB (React Native).

This of course includes dev dependencies. These do not get loaded during build-time and is actually a sizable chunk of that 900MB. At build-time I'd be looking at under 500 MB or less after minification. Size does not attribute to performance gains/losses, but rather how it utilizes the files in the app. It can be 60 GB large and still run smoothly.

It can effect the functionality of the app if there's a broken dependency, but it's your responsibility as a developer to work around those bugs. It would take you years of development to achieve similar functionality to that of React or similar frameworks using vanilla JS efficiently. Why waste all that time re-inventing the wheel? Ultimately such an attempt would actually be rendered pointless after a short time.

Webpack bundles more than just your dependencies. It bundles all your project files. It can also minify your files and makes everything considerable smaller. Small enough that no one would actually have anything to complain about if project footprint is an important factor.