Now available on the WordPress.org plugin directory: https://wordpress.org/plugins/lazy-loading-oembed-iframes/

.gitignore

.svn

readme.txt

=== Lazy Loading oEmbed Iframes ===
Contributors: westonruter
Tags: oembed, performance
Requires at least: 2.9
Tested up to: 5.4
Stable tag: trunk
License: GPLv2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Requires PHP: 5.6

Add the loading=lazy attribute to iframes coming from oEmbeds in content

== Description ==

This plugin adds the <code>loading=lazy</code> attribute to <code>iframe</code> elements coming from oEmbeds. In WordPress 5.5, <code>loading=lazy</code> is added to images but not yet iframes (though it was mentioned in <a href="https://core.trac.wordpress.org/ticket/44427">#44427</a>). For lazy-loaded images on WP 5.4 and before, use the <a href="https://wordpress.org/plugins/wp-lazy-loading/">Lazy Loading Feature Plugin</a>.

This plugin will likely be obsolete either by the time WordPress 5.5 is released or soon thereafter.

lazy-loading-oembed-iframes.php

<?php
/**
 * Lazy Loading oEmbed Iframes plugin bootstrap.
 *
 * @package   Google\Lazy_Loading_oEmbed_Iframes
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2020 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: Lazy Loading oEmbed Iframes
 * Plugin URI: https://gist.github.com/westonruter/9a8cda7307d08140f9edc29d246e46e8
 * Description: Add <code>loading=lazy</code> to <code>iframe</code> elements coming from oEmbeds. In WordPress 5.5, <code>loading=lazy</code> is added to images but not yet iframes (though it was mentioned in <a href="https://core.trac.wordpress.org/ticket/44427">#44427</a>). For lazy-loaded images on WP 5.4 and before, use the <a href="https://wordpress.org/plugins/wp-lazy-loading/">Lazy Loading Feature Plugin</a>.
 * Version: 1.0
 * Author: Weston Ruter
 * Author URI: https://weston.ruter.net/
 * License: GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * Gist Plugin URI: https://gist.github.com/westonruter/9a8cda7307d08140f9edc29d246e46e8
 */

namespace Google\Lazy_Loading_oEmbed_Iframes;

/**
 * Filter oEmbed HTML to inject loading=lazy attribute into iframes.
 *
 * @param string $html HTML for oEmbed.
 * @return string Filtered oEmbed HTML.
 */
function filter_oembed_html( $html ) {
	return preg_replace_callback(
		'/(?<=<iframe\s)[^>]+(?=>)/',
		function ( $matches ) {
			$attributes = $matches[0];
			if ( false === strpos( $attributes, 'loading=' ) ) {
				$attributes .= ' loading="lazy"';
			}
			return $attributes;
		},
		$html
	);
}

add_filter( 'embed_oembed_html', __NAMESPACE__ . '\filter_oembed_html' );

Installation instructions: https://gist.github.com/westonruter/6110fbc4bef0c4b8c021a112012f7e9c

This plugin has been added to the WordPress.org plugin directory. Install from there: https://wordpress.org/plugins/lazy-loading-oembed-iframes/