diff -u b/core/modules/user/src/Controller/UserController.php b/core/modules/user/src/Controller/UserController.php
--- b/core/modules/user/src/Controller/UserController.php
+++ b/core/modules/user/src/Controller/UserController.php
@@ -126,7 +126,11 @@
   }
 
   /**
-   * Redirects an authenticated user to its profile page.
+   * Redirects users to their profile page.
+   *
+   * This controller assumes that it is only invoked for authenticated users.
+   * This is enforced for the 'user.page' route with the '_user_is_logged_in'
+   * requirement.
    *
    * @return \Symfony\Component\HttpFoundation\RedirectResponse
    *   Returns a redirect to the profile of the currently logged in user.
diff -u b/core/modules/user/src/EventSubscriber/AccessDeniedSubscriber.php b/core/modules/user/src/EventSubscriber/AccessDeniedSubscriber.php
--- b/core/modules/user/src/EventSubscriber/AccessDeniedSubscriber.php
+++ b/core/modules/user/src/EventSubscriber/AccessDeniedSubscriber.php
@@ -63,8 +63,9 @@
    * {@inheritdoc}
    */
   public static function getSubscribedEvents() {
-    // Use a high priority here in order to beat core MaintenanceModeSubscriber
-    // as well as ExceptionLoggingSubscriber.
+    // Use a higher priority than
+    // \Drupal\Core\EventSubscriber\ExceptionLoggingSubscriber, because there's
+    // no need to log the exception if we can redirect.
     $events[KernelEvents::EXCEPTION][] = ['onException', 75];
     return $events;
   }
diff -u b/core/modules/user/src/EventSubscriber/MaintenanceModeSubscriber.php b/core/modules/user/src/EventSubscriber/MaintenanceModeSubscriber.php
--- b/core/modules/user/src/EventSubscriber/MaintenanceModeSubscriber.php
+++ b/core/modules/user/src/EventSubscriber/MaintenanceModeSubscriber.php
@@ -71,12 +71,12 @@
     }
     if ($this->account->isAuthenticated()) {
       if ($path == 'user/login') {
-        // If user is logged in, redirect to 'user' instead of giving 403.
+        // If the user is already logged in, redirect to their profile page.
         $event->setResponse($this->redirect('entity.user.canonical', ['user' => $this->account->id()]));
         return;
       }
       if ($path == 'user/register') {
-        // Authenticated user should be redirected to user edit page.
+        // If the user is already registered, redirect to their edit page.
         $event->setResponse(new RedirectResponse($this->url('entity.user.edit_form', ['user' => $this->account->id()], ['absolute' => TRUE])));
         return;
       }