Because it is what people see first, metrics calculated in the above-the-fold section means ensuring key content that is visible to the user is seen fast. Some content and support files can be loading in the background until you reach Fully loaded time. According to Koray Tuğberk GÜBÜR owner of Holistic SEO, In the relationship between loading CSS Files as Async and opening web pages earlier for the user, some Pagespeed Metrics are more positively affected than others. Faster loading of CSS Files especially improves First Paint, First Contentful Paint, Largest Contentful Paint, and Speed Index metrics.