.../EventSubscriber/ResourceResponseSubscriber.php |  5 +++--
 .../EntityResource/EntityResourceTestBase.php      | 25 ++++++++++++++++++++++
 .../src/EventSubscriber/AdminRouteSubscriber.php   | 20 ++++++++++++++---
 3 files changed, 45 insertions(+), 5 deletions(-)

diff --git a/core/modules/rest/src/EventSubscriber/ResourceResponseSubscriber.php b/core/modules/rest/src/EventSubscriber/ResourceResponseSubscriber.php
index 5cb887b..1223be0 100644
--- a/core/modules/rest/src/EventSubscriber/ResourceResponseSubscriber.php
+++ b/core/modules/rest/src/EventSubscriber/ResourceResponseSubscriber.php
@@ -196,8 +196,9 @@ protected function flattenResponse(ResourceResponseInterface $response) {
    * {@inheritdoc}
    */
   public static function getSubscribedEvents() {
-    // Run shortly before \Drupal\Core\EventSubscriber\FinishResponseSubscriber.
-    $events[KernelEvents::RESPONSE][] = ['onResponse', 5];
+    // Run before \Drupal\dynamic_page_cache\EventSubscriber\DynamicPageCacheSubscriber
+    // (priority 100), so that Dynamic Page Cache can cache flattened responses.
+    $events[KernelEvents::RESPONSE][] = ['onResponse', 128];
     return $events;
   }
 
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 53796c8..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;
@@ -386,6 +388,8 @@ public function testGet() {
     // 200 for well-formed HEAD request.
     $response = $this->request('HEAD', $url, $request_options);
     $this->assertResourceResponse(200, '', $response);
+    $this->assertTrue($response->hasHeader('X-Drupal-Dynamic-Cache'));
+    $this->assertSame(['MISS'], $response->getHeader('X-Drupal-Dynamic-Cache'));
     if (!$this->account) {
       $this->assertSame(['MISS'], $response->getHeader('X-Drupal-Cache'));
     }
@@ -395,13 +399,34 @@ public function testGet() {
     $head_headers = $response->getHeaders();
 
     // 200 for well-formed GET request. Page Cache hit because of HEAD request.
+    // Same for Dynamic Page Cache hit.
     $response = $this->request('GET', $url, $request_options);
     $this->assertResourceResponse(200, FALSE, $response);
+    $this->assertTrue($response->hasHeader('X-Drupal-Dynamic-Cache'));
     if (!static::$auth) {
       $this->assertSame(['HIT'], $response->getHeader('X-Drupal-Cache'));
+      $this->assertSame(['MISS'], $response->getHeader('X-Drupal-Dynamic-Cache'));
     }
     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));
diff --git a/core/modules/system/src/EventSubscriber/AdminRouteSubscriber.php b/core/modules/system/src/EventSubscriber/AdminRouteSubscriber.php
index b83fe7e..ac2a47f 100644
--- a/core/modules/system/src/EventSubscriber/AdminRouteSubscriber.php
+++ b/core/modules/system/src/EventSubscriber/AdminRouteSubscriber.php
@@ -4,10 +4,11 @@
 
 use Drupal\Core\Routing\RouteSubscriberBase;
 use Drupal\Core\Routing\RoutingEvents;
+use Symfony\Component\Routing\Route;
 use Symfony\Component\Routing\RouteCollection;
 
 /**
- * Adds the _admin_route option to each admin route.
+ * Adds the _admin_route option to each admin HTML route.
  */
 class AdminRouteSubscriber extends RouteSubscriberBase {
 
@@ -15,14 +16,27 @@ class AdminRouteSubscriber extends RouteSubscriberBase {
    * {@inheritdoc}
    */
   protected function alterRoutes(RouteCollection $collection) {
-    foreach ($collection->all() as $route) {
-      if (strpos($route->getPath(), '/admin') === 0 && !$route->hasOption('_admin_route')) {
+    foreach ($collection->all() as $name => $route) {
+      if (strpos($route->getPath(), '/admin') === 0 && !$route->hasOption('_admin_route') && static::isHtmlRoute($route)) {
         $route->setOption('_admin_route', TRUE);
       }
     }
   }
 
   /**
+   * Whether the given route is a HTML route.
+   *
+   * @param \Symfony\Component\Routing\Route $route
+   *   The route to inspect.
+   *
+   * @return bool
+   *   Whether the given route is a HTML route.
+   */
+  protected static function isHtmlRoute(Route $route) {
+    return !$route->hasRequirement('_format') || in_array('html', explode('|', $route->getRequirement('_format')), TRUE);
+  }
+
+  /**
    * {@inheritdoc}
    */
   public static function getSubscribedEvents() {