.../EntityResource/EntityResourceTestBase.php         | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 490d6a1..b01cd83 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -4,11 +4,13 @@
 
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\Cache;
+use Drupal\Core\Cache\CacheableResponseInterface;
 use Drupal\Core\Config\Entity\ConfigEntityInterface;
 use Drupal\Core\Entity\FieldableEntityInterface;
 use Drupal\Core\Url;
 use Drupal\field\Entity\FieldConfig;
 use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\rest\ResourceResponseInterface;
 use Drupal\Tests\rest\Functional\ResourceTestBase;
 use GuzzleHttp\RequestOptions;
 use Psr\Http\Message\ResponseInterface;
@@ -408,6 +410,23 @@ public function testGet() {
     else {
       $this->assertFalse($response->hasHeader('X-Drupal-Cache'));
       $this->assertSame(['HIT'], $response->getHeader('X-Drupal-Dynamic-Cache'));
+      // Assert that Dynamic Page Cache did not store a ResourceResponse object,
+      // which needs serialization after every cache hit. Instead, it should
+      // contain a flattened response. Otherwise performance suffers.
+      // @see \Drupal\rest\EventSubscriber\ResourceResponseSubscriber::flattenResponse()
+      $cache_items = $this->container->get('database')
+        ->query("SELECT cid, data FROM {cache_dynamic_page_cache} WHERE cid LIKE :pattern", [
+          ':pattern' => '%[route]=rest.%',
+        ])
+        ->fetchAllAssoc('cid');
+      foreach ($cache_items as $cid => $cache_item) {
+        $cached_data = unserialize($cache_item->data);
+        if (!isset($cached_data['#cache_redirect'])) {
+          $cached_response = $cached_data['#response'];
+          $this->assertNotInstanceOf(ResourceResponseInterface::class, $cached_response);
+          $this->assertInstanceOf(CacheableResponseInterface::class, $cached_response);
+        }
+      }
     }
     $cache_tags_header_value = $response->getHeader('X-Drupal-Cache-Tags')[0];
     $this->assertEquals($this->getExpectedCacheTags(), empty($cache_tags_header_value) ? [] : explode(' ', $cache_tags_header_value));