A simple Drupal input filter for ensuring hard-coded images include native lazy loading attributes.

FilterLazyLoad.php

<?php

namespace Drupal\chromatic_lazyload\Plugin\Filter;

use Drupal\Component\Utility\Html;
use Drupal\filter\FilterProcessResult;
use Drupal\filter\Plugin\FilterBase;

/**
 * Filter for altering hard-coded images to support native browser lazy loading.
 *
 * @Filter(
 *   id = "filter_lazyload",
 *   title = @Translation("Chromatic LazyLoad Filter"),
 *   description = @Translation("Updates hard-coded image element markup to include native lazy load attributes."),
 *   type = Drupal\filter\Plugin\FilterInterface::TYPE_TRANSFORM_REVERSIBLE,
 * )
 */
class FilterLazyLoad extends FilterBase {

  /**
   * {@inheritdoc}
   */
  public function process($text, $langcode) {
    $result = new FilterProcessResult($text);

    if (stristr($text, 'img') !== FALSE) {
      $dom = Html::load($text);
      $xpath = new \DOMXPath($dom);
      foreach ($xpath->query('//img') as $node) {
        // Set the loading attribute.
        $node->setAttribute('loading', 'lazy');
      }

      $result->setProcessedText(Html::serialize($dom));

    }

    return $result;

  }

}