How complex does a UI need to be to use React in an implementation rather than pure JS?

Question

JavaScript

Where is the line when you should not use libraries like React, but would it be better to use regular JS (ES > 6)?

Answer 1

I would rephrase the question: "When do I need to use 3rd party libraries instead of pure JS?". :)
Only when the increase in development speed (using third-party libraries) does not come at the expense of the performance of the code of the entire application.