Defer or Async CSS Loading

function async_css($tag, $handle) {

    // Add 'async' to CSS file that you want to load asynchronously

    if ('your-style-handle' !== $handle) {

        return $tag;

    }

    return str_replace(' href', ' async="async" href', $tag);

}

add_filter('style_loader_tag', 'async_css', 10, 2);