I have worked in ASP.NET CORE MVC, before.
I tried web search, I am unable to get proper answers.
What are all the advantages in using ReactJS over ASP.NET CORE?
ReactJS is high performance and rapid development client side framework, you can use it throw asp.net core or any other server side framework to handle client side operations.
Would not that additional library & its components, slow down the page load, comparing plain Razor Views?
ReactJS designed to be a single page application, and this kind of apps is more faster than multiple page applications, also ReactJS processing the DOM in different and smarter way.
If you want to use ReactJS in asp.net core you must keep in mind that asp.net core is the api part just to return the data (json or xml) or to receive your ajax request to post data.
All clients side parts will handled by ReactJS, no razor views, you can have only one to reference your reacts application or simply you can reference it using plain html page.