It's common that adding more layers of abstraction or indirection will make things slower. However, React and ClojureScript make web pages faster than doing it by hand -- essentially programming the bare web. The lesson is that if you choose your layers well, they can actually make your system faster.
React provides a better abstraction over the DOM than MVC frameworks ever can. React is the last piece of the puzzle for ClojureScript web frontend development.
React is a view library for web pages that makes DOM rendering in a functional style really easy. React makes web programming fun again.