diff --git a/lib/ClassBuilder.js b/lib/ClassBuilder.js
index b7d9d58..164fd86 100644
--- a/lib/ClassBuilder.js
+++ b/lib/ClassBuilder.js
@@ -1065,6 +1065,9 @@ function attachInstanceOf( instance )
         return module.exports.isInstanceOf( type, instance );
     };
 
+    // TODO: To improve performance (defineSecureProp can be costly), simply
+    // define a normal prop and freeze the class afterward. The class shouldn't
+    // have any mutable methods.
     util.defineSecureProp( instance, 'isInstanceOf', method );
     util.defineSecureProp( instance, 'isA', method );
 }