There is no doubt that CSS plays a huge role in web performance. Milica Mihajlija puts a point on exactly why:
When there is CSS available for a page, whether it’s inline or an external stylesheet, the browser delays rendering until the CSS is parsed. This is because pages without CSS are often unusable.
So how do you improve it? The classics like caching, minification, and compression help. But also, shipping less of it, and only loading the bit you need and the rest after the first render.
It’s entirely about how and how much CSS you load, and has very little to do with the contents of the the CSS.