single-post.php:

<img src="<?php the_post_thumbnail_url(\'medium\') ?>" class="img-fluid rounded shadow-3 mb-1" alt="<?php the_title() ?>" loading="lazy">

    add_filter( \'post_thumbnail_html\', \'wpdd_modify_post_thumbnail_html\', 10, 5 );
    /**
     * Add `loading="lazy"` attribute to images output by the_post_thumbnail().
     *
     * @param  string $html The post thumbnail HTML.
     * @param  int $post_id The post ID.
     * @param  string $post_thumbnail_id    The post thumbnail ID.
     * @param  string|array $size   The post thumbnail size. Image size or array of width and height values (in that order). Default \'post-thumbnail\'.
     * @param  string $attr Query string of attributes.
     * 
     * @return string   The modified post thumbnail HTML.
     */
    function wpdd_modify_post_thumbnail_html( $html, $post_id, $post_thumbnail_id, $size, $attr ) {
    
        return str_replace( \'<img\', \'<img loading="lazy"\', $html );
    
    }

source: https://wpdevdesign.com/native-lazy-loading-responsive-images-in-wordpress/