November 15th, 2013
If async is not present and defer is present: The script is executed when the page has finished parsingIf async is present: The script is executed asynchronously with the rest of the page (the script will be executed while the page continues the parsing)
You will see the difference with YSlow and Google Page speed ranking but your visitors will thank you for providing a speedy website.
– If neither async or defer is present: The script is fetched and executed immediately, before the browser continues parsing the page
This will improve your global page loading time
However, this is not applicable to all scripts as you want to make sure the script is loaded while some other script or inline script will call it.