diff --git a/lib/legacy/class.js b/lib/legacy/class.js
index 3c537c9b80..87ac046c73 100644
--- a/lib/legacy/class.js
+++ b/lib/legacy/class.js
@@ -235,6 +235,9 @@ function GenerateClassFromInfo(info, Base, behaviors) {
   /** @private */
   class PolymerGenerated extends Base {
 
+    // NOTE, this copies most of the code from DisableUpgradeMixin. This is
+    // unfortunate, but testing revealed a small penalty to applying the mixin
+    // on top of every generated class, so we do it directly here.
     static get observedAttributes() {
       return observedAttributesGetter.call(this).concat(DISABLED_ATTR);
     }