diff --git a/CHANGELOG.md b/CHANGELOG.md
index 79f30f27f..f7270b332 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,8 @@
 # Change Log
 
+## Unreleased
+- FlattenedNodesObserver.getFlattenedNodes was accidentally quadratic if given a non-slot node. Found by @mvanbem-goog.
+
 ## [v3.5.1](https://github.com/Polymer/polymer/tree/v3.5.1) (2022-06-03)
 - [ci skip] bump to 3.5.1 ([commit](https://github.com/Polymer/polymer/commit/2cbb3d2b))
 
diff --git a/lib/utils/flattened-nodes-observer.js b/lib/utils/flattened-nodes-observer.js
index 920ef9a6f..24a5156ba 100644
--- a/lib/utils/flattened-nodes-observer.js
+++ b/lib/utils/flattened-nodes-observer.js
@@ -88,14 +88,17 @@ export let FlattenedNodesObserver = class {
       node = /** @type {!HTMLSlotElement} */(node); // eslint-disable-line no-self-assign
       return wrapped.assignedNodes({flatten: true});
     } else {
-      return Array.from(wrapped.childNodes).map((node) => {
+      const results = [];
+      for (let i = 0; i < wrapped.childNodes.length; i++) {
+        const node = wrapped.childNodes[i];
         if (isSlot(node)) {
-          node = /** @type {!HTMLSlotElement} */(node); // eslint-disable-line no-self-assign
-          return wrap(node).assignedNodes({flatten: true});
+          const slotNode = /** @type {!HTMLSlotElement} */ (node);
+          results.push(...wrap(slotNode).assignedNodes({ flatten: true }));
         } else {
-          return [node];
+          results.push(node);
         }
-      }).reduce((a, b) => a.concat(b), []);
+      }
+      return results;
     }
   }
 
@@ -311,4 +314,4 @@ export let FlattenedNodesObserver = class {
     }
   }
 
-};
\ No newline at end of file
+};