To add "Defer" into the script

functions.php

<?php 
if(!function_exists('defer_parsing_of_js')) {
    function defer_parsing_of_js($url) {
        if (is_admin()) return $url; //don't break WP Admin
        if (false === strpos($url, '.js')) return $url;
        if (strpos($url, 'jquery.js')) return $url;
        return str_replace(' src', ' defer src', $url);
    }
}
add_filter('script_loader_tag', 'defer_parsing_of_js', 10);