diff --git a/core/lib/Drupal/Core/Routing/RouteBuilder.php b/core/lib/Drupal/Core/Routing/RouteBuilder.php
index 0dce99d..af60f2a 100644
--- a/core/lib/Drupal/Core/Routing/RouteBuilder.php
+++ b/core/lib/Drupal/Core/Routing/RouteBuilder.php
@@ -45,6 +45,13 @@ class RouteBuilder implements RouteBuilderInterface, DestructableInterface {
   protected $dispatcher;
 
   /**
+   * The yaml discovery used to find all the .routing.yml files.
+   *
+   * @var \Drupal\Component\Discovery\YamlDiscovery
+   */
+  protected $yamlDiscovery;
+
+  /**
    * The module handler.
    *
    * @var \Drupal\Core\Extension\ModuleHandlerInterface
@@ -173,6 +180,7 @@ public function rebuild() {
         $route = new Route($route_info['path'], $route_info['defaults'], $route_info['requirements'], $route_info['options']);
         $collection->add($name, $route);
       }
+
     }
 
     // DYNAMIC is supposed to be used to add new routes based upon all the
@@ -225,8 +233,10 @@ public function destruct() {
    *   The defined routes, keyed by provider.
    */
   protected function getRouteDefinitions() {
-    $discovery = new YamlDiscovery('routing', $this->moduleHandler->getModuleDirectories());
-    return $discovery->findAll();
+    if (!isset($this->yamlDiscovery)) {
+      $this->yamlDiscovery = new YamlDiscovery('routing', $this->moduleHandler->getModuleDirectories());
+    }
+    return $this->yamlDiscovery->findAll();
   }
 
 }