Critical css helps you reduce the rendering time.
Probably, your site has too many style codes and not all of them are used at first time. Because of that, they make to only serve the CSS that’s being used on that page and that’s commonly referred to as critical css. After making critical css, the download amount is certainly lower than before and it will improve performance too.
Enable Critical Css
There are many features to improve the performance and First Contentful Paint is one of the most important feature. So Generate critical css to reduce the rendering time and get a better speed.