diff --git a/CHANGELOG b/CHANGELOG
index 66ed574dc..080673f47 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,6 @@
 # 3.30.0 (2026-XX-XX)
 
+ * Speed up adding extensions to an environment
  * Fix the escaping safe analysis retaining every compiled template node for the lifetime of the environment
  * Speed up loading a template that the environment has already loaded
  * Speed up rendering by compiling a cheaper generator guard into templates
diff --git a/src/Environment.php b/src/Environment.php
index 105689902..c0fdfc32c 100644
--- a/src/Environment.php
+++ b/src/Environment.php
@@ -169,7 +169,7 @@ class Environment
     public function enableDebug()
     {
         $this->debug = true;
-        $this->updateOptionsHash();
+        $this->optionsHash = null;
     }
 
     /**
@@ -180,7 +180,7 @@ class Environment
     public function disableDebug()
     {
         $this->debug = false;
-        $this->updateOptionsHash();
+        $this->optionsHash = null;
     }
 
     /**
@@ -231,7 +231,7 @@ class Environment
     public function enableStrictVariables()
     {
         $this->strictVariables = true;
-        $this->updateOptionsHash();
+        $this->optionsHash = null;
     }
 
     /**
@@ -242,7 +242,7 @@ class Environment
     public function disableStrictVariables()
     {
         $this->strictVariables = false;
-        $this->updateOptionsHash();
+        $this->optionsHash = null;
     }
 
     /**
@@ -324,7 +324,7 @@ class Environment
      */
     public function getTemplateClass(string $name, ?int $index = null): string
     {
-        $key = ($this->hotCache[$name] ?? $this->getLoader()->getCacheKey($name)).$this->optionsHash;
+        $key = ($this->hotCache[$name] ?? $this->getLoader()->getCacheKey($name)).($this->optionsHash ??= $this->getOptionsHash());
 
         return '__TwigTemplate_'.hash(\PHP_VERSION_ID < 80100 ? 'sha256' : 'xxh128', $key).(null === $index ? '' : '___'.$index);
     }
@@ -701,7 +701,7 @@ class Environment
     public function addExtension(ExtensionInterface $extension)
     {
         $this->extensionSet->addExtension($extension);
-        $this->updateOptionsHash();
+        $this->optionsHash = null;
     }
 
     /**
@@ -712,7 +712,7 @@ class Environment
     public function setExtensions(array $extensions)
     {
         $this->extensionSet->setExtensions($extensions);
-        $this->updateOptionsHash();
+        $this->optionsHash = null;
     }
 
     /**
@@ -952,9 +952,9 @@ class Environment
         return $this->extensionSet->getExpressionParsers();
     }
 
-    private function updateOptionsHash(): void
+    private function getOptionsHash(): string
     {
-        $this->optionsHash = implode(':', [
+        return implode(':', [
             $this->extensionSet->getSignature(),
             \PHP_MAJOR_VERSION,
             \PHP_MINOR_VERSION,
diff --git a/tests/EnvironmentTest.php b/tests/EnvironmentTest.php
index 9c9818fbf..cd9511e86 100644
--- a/tests/EnvironmentTest.php
+++ b/tests/EnvironmentTest.php
@@ -655,6 +655,44 @@ EOF
             FilesystemHelper::removeDir($dir);
         }
     }
+
+    public function testTemplateClassFollowsExtensionsAddedAfterItWasComputed(): void
+    {
+        $twig = new Environment(new ArrayLoader(['index' => '']));
+        $cls = $twig->getTemplateClass('index');
+
+        $twig->addExtension(new EnvironmentTest_Extension_WithGlobals());
+        $this->assertNotSame($cls, $cls = $twig->getTemplateClass('index'));
+
+        $twig->setExtensions([new EnvironmentTest_ExtensionWithoutRuntime()]);
+        $this->assertNotSame($cls, $cls = $twig->getTemplateClass('index'));
+
+        $other = new Environment(new ArrayLoader(['index' => '']));
+        $other->setExtensions([new EnvironmentTest_Extension_WithGlobals(), new EnvironmentTest_ExtensionWithoutRuntime()]);
+        $this->assertSame($cls, $other->getTemplateClass('index'));
+    }
+
+    public function testTemplateClassFollowsDebugAndStrictVariablesChanges(): void
+    {
+        $twig = new Environment(new ArrayLoader(['index' => '']));
+        $cls = $twig->getTemplateClass('index');
+
+        $twig->enableDebug();
+        $debugCls = $twig->getTemplateClass('index');
+        $this->assertNotSame($cls, $debugCls);
+        $this->assertSame((new Environment(new ArrayLoader(['index' => '']), ['debug' => true]))->getTemplateClass('index'), $debugCls);
+
+        $twig->disableDebug();
+        $this->assertSame($cls, $twig->getTemplateClass('index'));
+
+        $twig->enableStrictVariables();
+        $strictCls = $twig->getTemplateClass('index');
+        $this->assertNotSame($cls, $strictCls);
+        $this->assertSame((new Environment(new ArrayLoader(['index' => '']), ['strict_variables' => true]))->getTemplateClass('index'), $strictCls);
+
+        $twig->disableStrictVariables();
+        $this->assertSame($cls, $twig->getTemplateClass('index'));
+    }
 }
 
 class EnvironmentTest_Extension_WithGlobals extends AbstractExtension