Dummy comment entity necessary for building comment form is expensive to build

Rendering the comment form for authenticated users happens via #post_render_cache, because the form is personalized for the current user. But, to build the comment form, we need a comment entity. And to build a comment entity, we must use ::create(), which will call ::applyDefaultValues(), which is expensive because of all the Typed Data things (IIRC) it needs in order to function.

  public function renderForm(array $element, array $context) {
    $field_name = $context['field_name'];
    $entity = $this->entityManager->getStorage($context['entity_type'])->load($context['entity_id']);
    $field_storage = FieldStorageConfig::loadByName($entity->getEntityTypeId(), $field_name);
    $values = array(
      'entity_type' => $entity->getEntityTypeId(),
      'entity_id' => $entity->id(),
      'field_name' => $field_name,
      'comment_type' => $field_storage->getSetting('bundle'),
      'pid' => NULL,
    );
    $comment = $this->entityManager->getStorage('comment')->create($values);
    $form = $this->entityFormBuilder->getForm($comment);
    $markup = $this->renderer->render($form);

    $callback = 'comment.post_render_cache:renderForm';
    $placeholder = $this->generatePlaceholder($callback, $context);
    $element['#markup'] = str_replace($placeholder, $markup, $element['#markup']);
    $element = $this->renderer->mergeBubbleableMetadata($element, $form);

    return $element;
  }

Applying the same trick as in #2458817: Creating new user entities for anonymous users is very slow is impossible, because it's very likely for comment entities to have additional fields, and because we are showing a form, we need those default values to be applied (so that their widgets have the expected default values)!