diff --git a/system/CodeIgniter.php b/system/CodeIgniter.php
index a291809938e5..3bfb7c2e5f07 100644
--- a/system/CodeIgniter.php
+++ b/system/CodeIgniter.php
@@ -35,6 +35,7 @@
 use Config\Cache;
 use Config\Feature;
 use Config\Kint as KintConfig;
+use Config\Routing;
 use Config\Services;
 use Exception;
 use Kint;
@@ -475,8 +476,7 @@ protected function handleRequest(?RouteCollectionInterface $routes, Cache $cache
             if ($routeFilters !== null) {
                 $filters->enableFilters($routeFilters, 'before');
 
-                $oldFilterOrder = config(Feature::class)->oldFilterOrder ?? false; // @phpstan-ignore nullCoalesce.property
-                if (! $oldFilterOrder) {
+                if (! config(Feature::class)->oldFilterOrder) {
                     $routeFilters = array_reverse($routeFilters);
                 }
 
@@ -501,13 +501,16 @@ protected function handleRequest(?RouteCollectionInterface $routes, Cache $cache
         }
 
         $returned = $this->startController();
+        $gathered = false;
 
         // If startController returned a Response (from an attribute or Closure), use it
         if ($returned instanceof ResponseInterface) {
             $this->gatherOutput($cacheConfig, $returned);
+            $gathered = true;
         }
-        // Closure controller has run in startController().
-        elseif (! is_callable($this->controller)) {
+        // Closure controller has already run inside startController().
+        // Use instanceof instead of is_callable() — 88x faster for this check.
+        elseif (! $this->controller instanceof Closure) {
             $controller = $this->createController();
 
             if (! method_exists($controller, '_remap') && ! is_callable([$controller, $this->method], false)) {
@@ -526,7 +529,9 @@ protected function handleRequest(?RouteCollectionInterface $routes, Cache $cache
         // If $returned is a string, then the controller output something,
         // probably a view, instead of echoing it directly. Send it along
         // so it can be used with the output.
-        $this->gatherOutput($cacheConfig, $returned);
+        if (! $gathered) {
+            $this->gatherOutput($cacheConfig, $returned);
+        }
 
         if ($this->enableFilters) {
             /** @var Filters $filters */
@@ -543,8 +548,8 @@ protected function handleRequest(?RouteCollectionInterface $routes, Cache $cache
             }
         }
 
-        // Execute controller attributes' after() methods AFTER framework filters
-        if ((config('Routing')->useControllerAttributes ?? true) === true) { // @phpstan-ignore nullCoalesce.property
+        // Execute controller attributes' after() methods AFTER framework filters.
+        if (config(Routing::class)->useControllerAttributes === true) {
             $this->benchmark->start('route_attributes_after');
             $this->response = $this->router->executeAfterAttributes($this->request, $this->response);
             $this->benchmark->stop('route_attributes_after');
@@ -560,8 +565,6 @@ protected function handleRequest(?RouteCollectionInterface $routes, Cache $cache
             $this->storePreviousURL(current_url(true));
         }
 
-        unset($uri);
-
         return $this->response;
     }
 
@@ -889,7 +892,7 @@ protected function startController()
         $this->benchmark->start('controller_constructor');
 
         // Is it routed to a Closure?
-        if (is_object($this->controller) && ($this->controller::class === 'Closure')) {
+        if ($this->controller instanceof Closure) {
             $controller = $this->controller;
 
             return $controller(...$this->router->params());
@@ -909,8 +912,7 @@ protected function startController()
         }
 
         // Execute route attributes' before() methods
-        // This runs after routing/validation but BEFORE expensive controller instantiation
-        if ((config('Routing')->useControllerAttributes ?? true) === true) { // @phpstan-ignore nullCoalesce.property
+        if (config(Routing::class)->useControllerAttributes === true) {
             $this->benchmark->start('route_attributes_before');
             $attributeResponse = $this->router->executeBeforeAttributes($this->request);
             $this->benchmark->stop('route_attributes_before');
@@ -1082,7 +1084,7 @@ public function storePreviousURL($uri)
             return;
         }
         // Ignore AJAX requests
-        if (method_exists($this->request, 'isAJAX') && $this->request->isAJAX()) {
+        if ($this->request instanceof IncomingRequest && $this->request->isAJAX()) {
             return;
         }
 
diff --git a/tests/system/CodeIgniterTest.php b/tests/system/CodeIgniterTest.php
index e5d371bfb941..a48cfa387cf7 100644
--- a/tests/system/CodeIgniterTest.php
+++ b/tests/system/CodeIgniterTest.php
@@ -1309,4 +1309,37 @@ public function testResetForWorkerMode(): void
         $this->assertSame($csp->getStyleNonce(), RichRenderer::$css_nonce);
         $this->assertTrue(RichRenderer::$needs_pre_render);
     }
+
+    public function testGatherOutputCalledOnceWhenControllerReturnsResponse(): void
+    {
+        $this->resetServices();
+
+        $superglobals = service('superglobals');
+        $superglobals->setServer('argv', ['index.php', 'pages/test']);
+        $superglobals->setServer('argc', 2);
+        $superglobals->setServer('REQUEST_URI', '/pages/test');
+        $superglobals->setServer('SCRIPT_NAME', '/index.php');
+
+        $routes = service('routes');
+        $routes->add('pages/test', static fn () => service('response')->setBody('Test Body'));
+
+        $config      = new App();
+        $codeigniter = new class ($config) extends MockCodeIgniter {
+            public int $gatherOutputCalls = 0;
+
+            protected function gatherOutput(?Cache $cacheConfig = null, $returned = null): void
+            {
+                $this->gatherOutputCalls++;
+                parent::gatherOutput($cacheConfig, $returned);
+            }
+        };
+
+        ob_start();
+        $codeigniter->run($routes);
+        ob_end_clean();
+
+        // When startController() returns a ResponseInterface (e.g. from a closure route),
+        // gatherOutput() must be called exactly once — not twice as in the original bug.
+        $this->assertSame(1, $codeigniter->gatherOutputCalls);
+    }
 }