.../lib/Drupal/comment/CommentViewBuilder.php      |   33 ++++++++++++++++++--
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/core/modules/comment/lib/Drupal/comment/CommentViewBuilder.php b/core/modules/comment/lib/Drupal/comment/CommentViewBuilder.php
index 0692599..68ef009 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentViewBuilder.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentViewBuilder.php
@@ -167,9 +167,36 @@ public function buildContent(array $entities, array $displays, $view_mode, $lang
   protected function getBuildDefaults(EntityInterface $entity, $view_mode, $langcode) {
     $defaults = parent::getBuildDefaults($entity, $view_mode, $langcode);
 
-    // Add the page number to the cache key if render caching is enabled.
-    if (isset($defaults['#cache']) && $this->request->query->has('page')) {
-      $defaults['#cache']['keys'][] = 'comment-pager-' . $this->request->query->get('page');
+    $entity_type = $entity->entity_type->value;
+    $entity_id = $entity->entity_id->value;
+    // We need to be able to access the "per_page" comment field instance
+    // setting, and unfortunately that means we need the commented entity's
+    // bundle, which is a piece of metadata that is not available on the Comment
+    // entity. Hence we must actually load the commented entity to figure out
+    // the bundle.
+    // @todo THIS IS BAD! Is there a reason why we shouldn't add a "bundle"
+    //       property (equal to the commented entity's bundle) on the Comment
+    //       entity?
+    $commented_entity = entity_load($entity_type, $entity_id);
+    $bundle = $commented_entity->bundle();
+    // A Comment entity's "field_id" property contains the commented entity type
+    // and the comment field name, separated by two underscores. To retrieve the
+    // actual field name, we must use string parsing.
+    // @todo THIS IS BAD! Is there a reason why we shouldn't add a "field_name"
+    //       proeprty (equal to the commented entity's comment field name) on
+    //       the Comment entity?
+    $field_name = substr($entity->field_id->value, strlen($entity_type) + 2);
+
+    // Retrieve the "per_page" comment field instance setting.
+    $instance = $this->fieldInfo->getInstance($entity_type, $bundle, $field_name);
+    $comments_per_page = $instance->getSetting('per_page');
+
+    // Add the comments per page and the page number to the cache key if render
+    // caching is enabled.
+    if (isset($defaults['#cache'])) {
+      $defaults['#cache']['keys'][] = 'comments-per-page-' . $comments_per_page;
+      $page = $this->request->query->has('page') ? $this->request->query->get('page') : 0;
+      $defaults['#cache']['keys'][] = 'comment-pager-' . $page;
     }
 
     return $defaults;