diff --git a/UPGRADE-3.4.md b/UPGRADE-3.4.md
index 00d6422f76..2638d84c80 100644
--- a/UPGRADE-3.4.md
+++ b/UPGRADE-3.4.md
@@ -72,6 +72,16 @@ DependencyInjection
  * The `ResolveDefinitionTemplatesPass` class is deprecated and will be removed in 4.0.
    Use the `ResolveChildDefinitionsPass` class instead.
 
+ * Unless you're using a custom autoloader, you should enable the `container.dumper.inline_class_loader`
+   parameter. This can drastically improve DX by reducing the time to load classes
+   when the `DebugClassLoader` is enabled. If you're using `FrameworkBundle`, this
+   performance improvement will also impact the "dev" environment:
+
+   ```yml
+   parameters:
+       container.dumper.inline_class_loader: true
+   ```
+
 Debug
 -----
 
diff --git a/UPGRADE-4.0.md b/UPGRADE-4.0.md
index 4eb1a0ed2a..4a6d02b374 100644
--- a/UPGRADE-4.0.md
+++ b/UPGRADE-4.0.md
@@ -236,6 +236,17 @@ DependencyInjection
 
  * The `ExtensionCompilerPass` has been moved to before-optimization passes with priority -1000.
 
+ * In 3.4, parameter `container.dumper.inline_class_loader` was introduced. Unless
+   you're using a custom autoloader, you should enable this parameter. This can
+   drastically improve DX by reducing the time to load classes when the `DebugClassLoader`
+   is enabled. If you're using `FrameworkBundle`, this performance improvement will
+   also impact the "dev" environment:
+
+   ```yml
+   parameters:
+       container.dumper.inline_class_loader: true
+   ```
+
 DoctrineBridge
 --------------