Help !!!! Javascript

as much as i know about Bootstrap, will make the page heavy when use it most, because it needs get its styling from somewhere so if the network is slow bootstrap will get effected, if it gets effected it makes the page heavy and slow

Nothing is faster than plain HTML + JS. The more layers you keep adding the slower its gonna get. I will say stick to HTML + BS + CSS if its a simple static page. If its a complex one then you can think of Adding framework and other stuff.

Maybe people in the comment are confused/mistook whether it's about 'fast in performance' or 'fast in development'.

No, There will be some kind of latency involved.

I think, it will be fast but not only BC of bootstrap but also react gives you some advantages.
First: when you made a website with HTML CSS and js, your server request each time if you try to upload New pages. But react doesnt do that, react built on an html page which is single and when you wanna change page it will only change js. So it is called as single page application.
Second: when you write html, CSS and js, you RE making the dom. But when you made it with react, you just need to tell your wishes to react and it will handle DOM in fastest way.
So it is really fast but as far as i know doesnt recommended for small applications because it is a bit complex to learn.