WordPress filter: Use async with JavaScript urls.
<?php
add_filter( 'clean_url', function ($url){
if ( strpos( $url, '#asyncload') === false )
return $url;
else if ( is_admin() )
return str_replace( '#asyncload', '', $url );
else
return str_replace( '#asyncload', '', $url )."' async='async";
}, 11, 1);