There are two things to consider: 1. How fast the server responds and how quickly the page contents (HTML, CSS, JS, etc) are downloaded. 2. How long the rende

There are two things to consider: 1. How fast the server responds and how quickly the page contents (HTML, CSS, JS, etc) are downloaded. 2. How long the rendering of the contents takes. Using blocking JS will slow this down. Using horribly complex CSS selectors would also affect the rendering. Google Analytics moved to non-blocking loading recently (it's different JS code), which I think is part of a bigger push within Google. Other services should do the same.