diff --git a/src/utils/toHierarchy.js b/src/utils/toHierarchy.js
index a0e7f32c17..3f9c70a09a 100644
--- a/src/utils/toHierarchy.js
+++ b/src/utils/toHierarchy.js
@@ -13,21 +13,20 @@
 export function toHierarchy(flatArray, getParentId) {
   /** @type {NodeLike[]} */
   const tree = [];
-
-  // Use Map for O(1) lookups.
   const childrenOf = new Map();
+  const itemsMap = new Map(flatArray.map(item => [item.id, item]));
 
   flatArray.forEach((item) => {
     const parentId = getParentId(item);
 
-    // Only create nodes array if we have children
+    // Only create nodes array if we have children.
     const children = childrenOf.get(item.id);
     if (children) {
       item.nodes = children;
     }
 
-    // Check if parentId exists in the flatArray.
-    const parentExists = parentId && flatArray.some((p) => p.id === parentId);
+    // Check if parentId exists using Map instead of array lookup.
+    const parentExists = parentId && itemsMap.has(parentId);
 
     if (parentId && parentExists) {
       if (!childrenOf.has(parentId)) {
@@ -35,8 +34,7 @@ export function toHierarchy(flatArray, getParentId) {
       }
       childrenOf.get(parentId).push(item);
 
-      // If parent already processed, add nodes array.
-      const parent = flatArray.find((p) => p.id === parentId);
+      const parent = itemsMap.get(parentId);
       if (parent) {
         parent.nodes = childrenOf.get(parentId);
       }
diff --git a/tests/App.test.svelte b/tests/App.test.svelte
index de04ad2fe1..96c876d1f2 100644
--- a/tests/App.test.svelte
+++ b/tests/App.test.svelte
@@ -1,6 +1,7 @@
 <script lang="ts">
   import { TreeView as TreeViewNav } from "carbon-components-svelte";
   import TreeView from "./TreeView/TreeView.test.svelte";
+  import TreeViewHierarchy from "./TreeView/TreeView.hierarchy.test.svelte";
   import { onMount } from "svelte";
 
   const routes = [
@@ -9,6 +10,11 @@
       name: "TreeView",
       component: TreeView,
     },
+    {
+      path: "/treeview-hierarchy",
+      name: "TreeViewHierarchy",
+      component: TreeViewHierarchy,
+    },
   ] as const;
 
   let currentPath = window.location.pathname;
diff --git a/tests/TreeView/to-hierarchy.test.ts b/tests/TreeView/toHierarchy.test.ts
similarity index 100%
rename from tests/TreeView/to-hierarchy.test.ts
rename to tests/TreeView/toHierarchy.test.ts