PageSpeed: Prefer asynchronous resources
Fetching resources asynchronously prevents those resources from blocking the page load.
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Details from Google
When a browser parses a traditional script tag, it must wait for the script to download, parse, and execute before rendering any HTML that comes after it. With an asynchronous script, however, the browser can continue parsing and rendering HTML that comes after the async script, without waiting for that script to complete. When a script is loaded asynchronously, it is fetched as soon as possible, but its execution is deferred until the browser's UI thread is not busy doing something else, such as rendering the web page.