WordPress - Regenerate thumbnails in batches (faster)

/* DISCLAIMER
 * This script is deliberately simple. It doesn't check permissions, nor it
 * prevents users from triggering the regeneration multiple times in a row.
 * USE IT AT YOUR OWN RISK.
 *
 * Need help customising the code for your need? Hire us on Codeable: http://bit.ly/codeable_aelia
 */

/**
 * Regenerates the thumbnails for an image. Code inspired by the Regenerate
 * Thumbnails plugin (https://wordpress.org/plugins/regenerate-thumbnails/).
 *
 * @param int id An image ID.
 * @return array An array with the result of the operation.
 */
function aelia_process_image($id) {
  $image = get_post($id);

  if(!$image || 'attachment' != $image->post_type || 'image/' != substr($image->post_mime_type, 0, 6)) {
    return array(
      'message' => sprintf(__('Failed resize: %s is an invalid image ID.', 'regenerate-thumbnails'), esc_html($id))
    );
  }

  $fullsizepath = get_attached_file($image->ID);

  if(false === $fullsizepath || !file_exists($fullsizepath)) {
    return array(
      'message' => sprintf(__('Image ID: %s. The originally uploaded image file cannot be found at %s', 'regenerate-thumbnails'), $image->ID, esc_html($fullsizepath))
    );
  }

  $metadata = wp_generate_attachment_metadata($image->ID, $fullsizepath);

  if(is_wp_error($metadata)) {
    return array(
      'message' => sprintf('Image ID: %s.', $image->ID) . ' ' . $metadata->get_error_message(),
    );
  }
  if(empty($metadata)) {
    return array(
      'message' => sprintf('Image ID: %s. Unknown failure reason', $image->ID),
    );
  }

  // If this fails, then it just means that nothing was changed(old value == new value)
  wp_update_attachment_metadata($image->ID, $metadata);

  return array(
    'message' => sprintf(__('"%1$s"(ID %2$s) was successfully resized.', 'regenerate-thumbnails'),
                         esc_html(get_the_title($image->ID)),
                         $image->ID));
}

/**
 * Regenerates the thumbnails images when the Admin section is loaded. Trigger
 * the resizing by calling the following URL:
 * http://yoursite.com/wp-admin?regen_thumbs=1&img_id_from=<image ID>&img_id_to=<image ID>
 */
add_action('admin_init', function() {
  if(!empty($_GET['regen_thumbs']) && !empty($_GET['img_id_from'])) {
    $image_id_from = $_GET['img_id_from'];
    $image_id_to = !empty($_GET['img_id_to']) ? $_GET['img_id_to'] : $image_id_from + 40;

    set_time_limit(abs($image_id_to - $image_id_from) * 30); // 30 seconds per image should be PLENTY

    // Show all errors, to make it easier to troubleshoot issues
    error_reporting(E_ALL);
    ini_set('display_errors', 1);

    echo '<pre>';
    $time_start = microtime(true);
    echo "Started at " . date('Y-m-d H:i:s') . "\n";
    echo "Resizing images from $image_id_from to $image_id_to\n";
    for($image_id = $image_id_from; $image_id <= $image_id_to; $image_id++) {
      $result = aelia_process_image($image_id);
      if(isset($result['message'])) {
        echo $result['message'] . "\n";
      }
      else {
        echo sprintf("Invalid result received processing image ID %s", $image_id);
      }

      // Flush cache regularly, to avoid out of memory errors
      if($image_id % 10 == 0) {
        wp_cache_flush();
      }
    }
    $time_end = microtime(true);
    $execution_time = ($time_end - $time_start);
    echo "Finished at " . date('Y-m-d H:i:s') . "after $execution_time seconds";
    echo '</pre>';
  }
}, 20);