Add Defer & Async Attributes to many WordPress Script

Add this to the functions.php

function add_defer_attribute($tag, $handle) {
   // add script handles to the array below
   $scripts_to_defer = array('jquery-migrate', 'hoverIntent', 'superfish', 'superfish-args', 'skip-links', 'wp-embed');
   
   foreach($scripts_to_defer as $defer_script) {
      if ($defer_script === $handle) {
         return str_replace(' src', ' defer="defer" src', $tag);
      }
   }
   return $tag;
}
add_filter('script_loader_tag', 'add_defer_attribute', 10, 2);

//* If you want to use defer just replace async="async" with defer="defer".
//* You will need to change the handle which will be the first parameter of the enqueue method.

Full article: : https://www.evemilano.com/2017/10/aggiungere-defer-async/