Preload featured image in WordPress

preload-featured.php

function preload_featured_image() {
	if (has_post_thumbnail()) {
		$attachment_image = wp_get_attachment_url(get_post_thumbnail_id());
		if ($attachment_image) {
			echo '<link rel="preload" fetchpriority="high" as="image" href="' . esc_url($attachment_image) . '">';
		}
	}
}
add_action('wp_head', 'preload_featured_image');