add_preload_featured_image

php

function add_preload_featured_image() {
    if (is_single() || is_page()) {
        if (has_post_thumbnail()) {
            $post_thumbnail_id = get_post_thumbnail_id();
            $post_thumbnail_url = wp_get_attachment_image_url($post_thumbnail_id, 'full');

            if ($post_thumbnail_url) {
                echo '<link rel="preload" href="' . esc_url($post_thumbnail_url) . '" as="image" type="image/webp">';
            }
        }
    }
}
add_action('wp_head', 'add_preload_featured_image');