core/lib/Drupal/Core/Form/FormBuilder.php | 6 ++++++
 core/modules/comment/src/CommentForm.php  | 4 ++++
 core/modules/contact/src/MessageForm.php  | 4 ++++
 3 files changed, 14 insertions(+)

diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 6f31650..51ff254 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -770,6 +770,12 @@ public function prepareForm($form_id, &$form, FormStateInterface &$form_state) {
             ]
           ],
         );
+
+        // By default, make the form uncacheable. Allow forms to opt in to be
+        // cached by specifying their own max-age at the top level of the form.
+        if (!isset($form['#cache']['max-age'])) {
+          $form['form_token']['#cache']['max-age'] = 0;
+        }
       }
     }
 
diff --git a/core/modules/comment/src/CommentForm.php b/core/modules/comment/src/CommentForm.php
index 51a6cce..8b7ea66 100644
--- a/core/modules/comment/src/CommentForm.php
+++ b/core/modules/comment/src/CommentForm.php
@@ -10,6 +10,7 @@
 use Drupal\comment\Plugin\Field\FieldType\CommentItemInterface;
 use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\Unicode;
+use Drupal\Core\Cache\Cache;
 use Drupal\Core\Datetime\DrupalDateTime;
 use Drupal\Core\Entity\ContentEntityForm;
 use Drupal\Core\Entity\EntityConstraintViolationListInterface;
@@ -69,6 +70,9 @@ public function __construct(EntityManagerInterface $entity_manager, AccountInter
    * {@inheritdoc}
    */
   public function form(array $form, FormStateInterface $form_state) {
+    // This form is cacheable!
+    $form['#cache']['max-age'] = Cache::PERMANENT;
+
     /** @var \Drupal\comment\CommentInterface $comment */
     $comment = $this->entity;
     $entity = $this->entityManager->getStorage($comment->getCommentedEntityTypeId())->load($comment->getCommentedEntityId());
diff --git a/core/modules/contact/src/MessageForm.php b/core/modules/contact/src/MessageForm.php
index ac435ae..a56fdf9 100644
--- a/core/modules/contact/src/MessageForm.php
+++ b/core/modules/contact/src/MessageForm.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\contact;
 
+use Drupal\Core\Cache\Cache;
 use Drupal\Core\Datetime\DateFormatterInterface;
 use Drupal\Core\Entity\ContentEntityForm;
 use Drupal\Core\Entity\EntityManagerInterface;
@@ -94,6 +95,9 @@ public static function create(ContainerInterface $container) {
    * {@inheritdoc}
    */
   public function form(array $form, FormStateInterface $form_state) {
+    // This form is cacheable!
+    $form['#cache']['max-age'] = Cache::PERMANENT;
+
     $user = $this->currentUser();
     $message = $this->entity;
     $form = parent::form($form, $form_state, $message);