// Automatically add loading="lazy" to all img tags that lack a loading attribute
add_filter('the_content', 'add_lazy_loading_to_content_images', 99);
function add_lazy_loading_to_content_images($content) {
    if (is_admin()) {
        return $content;
    }

    return preg_replace_callback('/<img([^>]+)>/i', function($matches) {
        $img = $matches[0];

        // Skip if loading attribute is already defined or if fetchpriority="high" is set
        if (strpos($img, 'loading=') !== false || strpos($img, 'fetchpriority="high"') !== false) {
            return $img;
        }

        // Inject loading="lazy"
        return str_replace('<img', '<img loading="lazy"', $img);
    }, $content);
}