Issue #2099133 by Wim Leers: Comment form on same page as comments forces node render caching to be per user.

core/modules/comment/comment.module

@@ -1731,3 +1731,20 @@ function comment_library_info() {
   );
   return $libraries;
 }
+
+/**
+ * #post_render_cache callback; replaces the placeholder with the comment form.
+ *
+ * @param array $context
+ *   An array with the following keys:
+ *   - entity_type: an entity type
+ *   - entity_id: an entity ID
+ *   - field_name: a comment field name
+ *
+ * @return array $element
+ *   The updated $element.
+ */
+function comment_replace_form_placeholder(array $context) {
+  $entity = entity_load($context['entity_type'], $context['entity_id']);
+  return comment_add($entity, $context['field_name']);
+}

core/modules/comment/lib/Drupal/comment/Plugin/Field/FieldFormatter/CommentDefaultFormatter.php

@@ -138,8 +138,25 @@ class CommentDefaultFormatter extends FormatterBase implements ContainerFactoryP
       if ($status == COMMENT_OPEN && $comment_settings['form_location'] == COMMENT_FORM_BELOW) {
         // Only show the add comment form if the user has permission.
         if ($this->currentUser->hasPermission('post comments')) {
+          // All users in the "anonymous" role can use the same form: it is fine
+          // for this form to be stored in the render cache.
+          if ($this->currentUser->isAnonymous()) {
             $output['comment_form'] = comment_add($entity, $field_name);
           }
+          // All other users need a user-specific form, which would break the
+          // render cache: hence use a #post_render_cache callback.
+          else {
+            $output['comment_form'] = array(
+              '#type' => 'render_cache_placeholder',
+              '#callback' => 'comment_replace_form_placeholder',
+              '#context' => array(
+                'entity_type' => $entity->entityType(),
+                'entity_id' => $entity->id(),
+                'field_name' => $field_name
+              ),
+            );
+          }
+        }
       }
 
       $elements[] = $output + array(