diff --git a/.jules/bolt.md b/.jules/bolt.md
index dd1b466..f20f6da 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -21,3 +21,7 @@
 ## 2026-03-19 - FileManager Enumerator Pre-fetching
 **Learning:** Any property requested via `resourceValues(forKeys:)` inside a `FileManager.enumerator` loop must also be in `includingPropertiesForKeys` — otherwise `URL.resourceValues` falls back to a synchronous `stat()` per file, turning bulk reads into O(N) disk I/O.
 **Action:** Keep the keys array passed to `resourceValues(forKeys:)` a subset of the prefetch list passed to `FileManager.enumerator(at:includingPropertiesForKeys:)`.
+
+## 2024-06-25 - O(1) Index Maps for @Published Array Toggles
+**Learning:** When toggling the selection state of an item within a massive `@Published` array (like `nodeModulesItems`), sequentially searching for the element's index via `firstIndex(where:)` causes O(n) CPU latency on every user click.
+**Action:** Replace `firstIndex(where:)` with an O(1) dictionary-based index map (`[UUID: Int]`). Rebuild the dictionary via `enumerated().reduce(into: [:])` immediately after the main array is fully populated (e.g., after a scan operation completes), and include a defensive boundary and ID check fallback for robustness.
diff --git a/Sources/Cacheout/ViewModels/CacheoutViewModel.swift b/Sources/Cacheout/ViewModels/CacheoutViewModel.swift
index 0e8464b..60998b7 100644
--- a/Sources/Cacheout/ViewModels/CacheoutViewModel.swift
+++ b/Sources/Cacheout/ViewModels/CacheoutViewModel.swift
@@ -54,6 +54,9 @@ class CacheoutViewModel: ObservableObject {
     @Published var nodeModulesItems: [NodeModulesItem] = []
     @Published var isNodeModulesScanning = false
 
+    private var scanResultIndexMap: [UUID: Int] = [:]
+    private var nodeModulesIndexMap: [UUID: Int] = [:]
+
     /// Increments on every completed scan — views can use .task(id:) to react
     @Published var scanGeneration: Int = 0
 
@@ -154,9 +157,11 @@ class CacheoutViewModel: ObservableObject {
         async let nmResults = nodeModulesScanner.scan()
 
         scanResults = await cacheResults
+        scanResultIndexMap = scanResults.enumerated().reduce(into: [:]) { $0[$1.element.id] = $1.offset }
         isScanning = false
 
         nodeModulesItems = await nmResults
+        nodeModulesIndexMap = nodeModulesItems.enumerated().reduce(into: [:]) { $0[$1.element.id] = $1.offset }
         isNodeModulesScanning = false
 
         // Track scan completion for reactive UI updates
@@ -166,7 +171,9 @@ class CacheoutViewModel: ObservableObject {
     }
 
     func toggleSelection(for id: UUID) {
-        if let index = scanResults.firstIndex(where: { $0.id == id }) {
+        if let index = scanResultIndexMap[id], index < scanResults.count, scanResults[index].id == id {
+            scanResults[index].isSelected.toggle()
+        } else if let index = scanResults.firstIndex(where: { $0.id == id }) {
             scanResults[index].isSelected.toggle()
         }
     }
@@ -191,7 +198,9 @@ class CacheoutViewModel: ObservableObject {
     // MARK: - Node Modules selection
 
     func toggleNodeModulesSelection(for id: UUID) {
-        if let i = nodeModulesItems.firstIndex(where: { $0.id == id }) {
+        if let i = nodeModulesIndexMap[id], i < nodeModulesItems.count, nodeModulesItems[i].id == id {
+            nodeModulesItems[i].isSelected.toggle()
+        } else if let i = nodeModulesItems.firstIndex(where: { $0.id == id }) {
             nodeModulesItems[i].isSelected.toggle()
         }
     }