This feature creates critical CSS for your website to improve page load performance. Just click on the button, it will automatically crawl almost all the pages and create critical css on its own. Critical css generation is pretty fast like 10-15 mins but may take more time with site with thousand of pages.
In the Free Version, critical CSS will be generated only for the homepage. With the Pro Version (activated license key), unlimited critical CSS can be generated for your entire site.
CHT10