Drupal 8 lazy builder block

<?php

namespace Drupal\custom_module\Plugin\Block;

use Drupal\Core\Block\BlockBase;

/**
 * Provides a 'LazyBlock' block.
 *
 * @Block(
 *  id = "lazy_block",
 *  admin_label = @Translation("Lazy block"),
 * )
 */
class LazyBlock extends BlockBase {

  /**
   * {@inheritdoc}
   */
  public function build() {
    $build = [];
    $build['lazy_block']['#lazy_builder'] = [static::class . '::lazyMethod', ['argument1']];
    $build['lazy_block']['#create_placeholder'] = TRUE;

    return $build;
  }

  public static function lazyMethod($argument1) {
      sleep(5);
      return [
          'output' => [
              '#markup' => '<p>Well, this took a while.</p>'
          ]
      ];
  }


}