Preload a Chosen Hero Image per Page

// Post types that get the field.
$wpide_lcp_types = array( 'post', 'page' );

add_action( 'add_meta_boxes', function () use ( $wpide_lcp_types ) {
    add_meta_box( 'wpide-lcp-preload', 'Preload hero image', function ( $post ) {
        wp_nonce_field( 'wpide_lcp_preload', 'wpide_lcp_nonce' );
        printf(
            '<label for="wpide-lcp-url" class="screen-reader-text">Hero image URL</label><input type="url" id="wpide-lcp-url" name="wpide_lcp_url" class="widefat" value="%s" placeholder="https://..."><p class="description">URL of the large image at the top of this page.</p>',
            esc_attr( get_post_meta( $post->ID, '_wpide_lcp_url', true ) )
        );
    }, $wpide_lcp_types, 'side', 'low' );
} );

add_action( 'save_post', function ( $post_id ) {
    if ( ! isset( $_POST['wpide_lcp_nonce'] ) || ! wp_verify_nonce( sanitize_key( $_POST['wpide_lcp_nonce'] ), 'wpide_lcp_preload' ) || ! current_user_can( 'edit_post', $post_id ) ) {
        return;
    }

    $url = isset( $_POST['wpide_lcp_url'] ) ? esc_url_raw( wp_unslash( $_POST['wpide_lcp_url'] ) ) : '';

    if ( $url ) {
        update_post_meta( $post_id, '_wpide_lcp_url', $url );
    } else {
        delete_post_meta( $post_id, '_wpide_lcp_url' );
    }
} );

add_filter( 'wp_preload_resources', function ( $resources ) {
    if ( ! is_singular() ) {
        return $resources;
    }

    $url = get_post_meta( get_queried_object_id(), '_wpide_lcp_url', true );

    if ( $url ) {
        $resources[] = array( 'href' => $url, 'as' => 'image', 'fetchpriority' => 'high' );
    }

    return $resources;
} );