Speeding Up Async Snippets
For all the resulting script is asynchronous, the
<script>block that creates it is fully synchronous, which means that the discovery of the script is governed by any and all synchronous work that happens before it, whether that’s other synchronous JS, HTML, or even CSS. Effectively, we’ve hidden the file from the browser until the very last moment, which means we’re completely failing to take advantage of one of the browser’s most elegant internals… the Preload Scanner.
Is Progressive Enhancement Dead Yet?
Caching Header Best Practices