Async
Async / Defer Script Loading
By default, all scripts loading on a website are downloaded and executed in a synchronous manner. These means that if you have many and/or big scripts loading they can block each other and the page in general whilst they are downloading and executing. This can be very detrimental to the user experience and is particularly wasteful if the scripts in question are not needed for the first content draw. It iss preferred to load scripts in parallel with the rest of the document and assets.