It’s a widely known fact that page speed is an important factor in most search engines’ ranking algorithms, Google has categorically stated as much in their developer guidelines.
Google’s goal is to provide users with the most relevant results and a great user experience. Fast sites increase user satisfaction and improve the overall quality of the web
Plus, we are all instinctively aware that visiting a site which loads faster is a much more pleasant experience than one which takes several seconds. A factor that should make page speed a priority regardless of any benefit to search engine ranking.
Scripts block the rendering of a page until each has been parsed in turn by the browser. Therefore, deferring the loading of non-essential scripts until after the page has loaded is a critical step in optimising a users experience of your site.