Why do you think bad developers or teams without sufficient budget may produce bad experiences when using React? What should those teams be caring about that React doesn't solve for them?
Interactive UI development just isn't easy nor simple regardless of the platform and framework.
A tool like React can shrink the space you need to be vigilant about to ensure performance (not just speed but also UX), but you still need vigilance and attention to detail to make sure you're not making mistakes like running something on every re-render that you meant to do just once.
And it has its own idiosyncrasies that you need to be vigilant about. Like if you write a hook `const x = useMyHook({ optionA, optionB })` then you need to handle the cases (either at the call site or inside the hook) where the options object is new one every render and when optionA or optionB are non-primitives that change every render.
Also, website aren't slow because of React, they are slow because of shitty developers or insufficient budgets.