Generate Critical and Used CSS
Most of the plugins and themes inject their CSS files as render-blocking. The browser has to wait until these files are downloaded in order to render the page.
Critical and Used CSS helps to render the page immediately by deprioritizing the rest of the CSS files.
Critical CSS - CSS required for rendering of above fold. It's injected directly in the head tag as
Used CSS - CSS generated by removing all unused CSS. Injected as an external stylesheet that will be loaded asynchronously.