diff --git a/doc/project/module.md b/doc/project/module.md
index 63d5252..30bacd2 100644
--- a/doc/project/module.md
+++ b/doc/project/module.md
@@ -19,6 +19,8 @@ scalaJSLinkerConfig ~= { _.withModuleKind(ModuleKind.ESModule) }
 scalaJSLinkerConfig ~= { _.withModuleKind(ModuleKind.CommonJSModule) }
 {% endhighlight %}
 
+Note: when using ECMAScript modules, the [optimizations performed in `fullLinkJS`](../internals/performance.html) are limited, because the Google Closure Compiler cannot be used with them.
+
 When emitting a module, `@JSExportTopLevel`s are really *exported* from the Scala.js module.
 Moreover, you can use top-level `@JSImport` to [import native JavaScript stuff](../interoperability/facade-types.html#import) from other JavaScript module.