Wordpress preload featured image (src and srcset)

wordpress_preload_featured_image.php

<?php
// Preload the Wordpress post featured image (works both with src and srcset)
// ref. https://web.dev/preload-responsive-images/

add_action('wp_head', function () {
	if (has_post_thumbnail()) {
		$thumb_id = get_post_thumbnail_id( get_the_ID() );
		$featured_img_src = wp_get_attachment_image_src( $thumb_id, 'large' )[0];
		$featured_img_srcset = wp_get_attachment_image_srcset( $thumb_id );
		printf('<link rel="preload" as="image" importance="high" href="%s" srcset="%s" />', $featured_img_src, $featured_img_srcset );
	}
}, 1);