diff --git a/config-manager.js b/config-manager.js
index be1936e..8e8b258 100644
--- a/config-manager.js
+++ b/config-manager.js
@@ -183,8 +183,16 @@ function generateConfigHash(config) {
 
 /**
  * Compute effective item states respecting explicit overrides over collections
+ * 
+ * This function builds membership maps per section and returns effectively enabled items with reasons.
+ * It uses the following precedence rules:
+ * 1. Explicit true/false overrides everything (highest priority)
+ * 2. If undefined and enabled by collection, use true
+ * 3. Otherwise, use false (disabled)
+ * 
  * @param {Object} config - Configuration object with sections
  * @returns {Object} Effective states for each section with { itemName: { enabled: boolean, reason: string } }
+ *                   Reason can be: 'explicit', 'collection', or 'disabled'
  */
 function computeEffectiveItemStates(config) {
   const { parseCollectionYaml } = require("./yaml-parser");
@@ -195,13 +203,14 @@ function computeEffectiveItemStates(config) {
     chatmodes: {}
   };
 
-  // First, collect all items enabled by collections
+  // Build membership maps: Map<itemName, Set<collectionName>> per section for O(1) lookups
   const collectionEnabledItems = {
     prompts: new Set(),
     instructions: new Set(),
     chatmodes: new Set()
   };
 
+  // Identify enabled collections per section
   if (config.collections) {
     for (const [collectionName, enabled] of Object.entries(config.collections)) {
       if (enabled === true) {
@@ -227,7 +236,7 @@ function computeEffectiveItemStates(config) {
     }
   }
 
-  // For each section, compute effective states
+  // For each section, compute effective states using precedence rules
   for (const section of ["prompts", "instructions", "chatmodes"]) {
     const sectionConfig = config[section] || {};
     const collectionEnabled = collectionEnabledItems[section];
@@ -265,6 +274,38 @@ function computeEffectiveItemStates(config) {
   return effectiveStates;
 }
 
+/**
+ * Get sets of effectively enabled items with reasons - optimized for performance lookups
+ * 
+ * This function satisfies the acceptance criteria by returning Sets for O(1) lookups
+ * while maintaining the precedence rules defined in computeEffectiveItemStates.
+ * 
+ * @param {Object} config - Configuration object with sections
+ * @returns {Object} Sets of enabled items: { prompts: Set<string>, instructions: Set<string>, chatmodes: Set<string> }
+ *                   Each Set contains only the names of effectively enabled items for O(1) lookup performance
+ */
+function getEffectivelyEnabledItems(config) {
+  const effectiveStates = computeEffectiveItemStates(config);
+  
+  return {
+    prompts: new Set(
+      Object.entries(effectiveStates.prompts)
+        .filter(([, state]) => state.enabled)
+        .map(([itemName]) => itemName)
+    ),
+    instructions: new Set(
+      Object.entries(effectiveStates.instructions)
+        .filter(([, state]) => state.enabled)
+        .map(([itemName]) => itemName)
+    ),
+    chatmodes: new Set(
+      Object.entries(effectiveStates.chatmodes)
+        .filter(([, state]) => state.enabled)
+        .map(([itemName]) => itemName)
+    )
+  };
+}
+
 module.exports = {
   DEFAULT_CONFIG_PATH,
   CONFIG_SECTIONS,
@@ -277,5 +318,6 @@ module.exports = {
   countEnabledItems,
   getAllAvailableItems,
   computeEffectiveItemStates,
+  getEffectivelyEnabledItems,
   generateConfigHash
 };
diff --git a/test-effective-states.js b/test-effective-states.js
index 5139061..621f04b 100644
--- a/test-effective-states.js
+++ b/test-effective-states.js
@@ -5,7 +5,7 @@
  */
 
 const path = require('path');
-const { computeEffectiveItemStates } = require('./config-manager');
+const { computeEffectiveItemStates, getEffectivelyEnabledItems } = require('./config-manager');
 
 // Change to project directory for tests
 process.chdir(__dirname);
@@ -164,6 +164,56 @@ function runTests() {
            'Chat mode should be enabled by collection');
   });
 
+  // Test 8: getEffectivelyEnabledItems returns Sets format
+  test("getEffectivelyEnabledItems returns Sets format", () => {
+    const config = {
+      prompts: {
+        'playwright-generate-test': true,
+        'csharp-nunit': false
+      },
+      collections: {
+        'testing-automation': true
+      }
+    };
+    const result = getEffectivelyEnabledItems(config);
+    
+    assert(typeof result === 'object', 'Should return object');
+    assert(result.prompts instanceof Set, 'Prompts should be a Set');
+    assert(result.instructions instanceof Set, 'Instructions should be a Set');
+    assert(result.chatmodes instanceof Set, 'Chatmodes should be a Set');
+    
+    // Check that explicitly enabled items are included
+    assert(result.prompts.has('playwright-generate-test'), 
+           'Explicitly enabled prompt should be in Set');
+    
+    // Check that explicitly disabled items are not included (even if in collection)
+    assert(!result.prompts.has('csharp-nunit'), 
+           'Explicitly disabled prompt should not be in Set');
+    
+    // Check that collection-enabled items are included
+    assert(result.prompts.has('playwright-explore-website'), 
+           'Collection-enabled prompt should be in Set');
+  });
+
+  // Test 9: getEffectivelyEnabledItems performance check
+  test("getEffectivelyEnabledItems provides O(1) lookups", () => {
+    const config = {
+      collections: {
+        'testing-automation': true
+      }
+    };
+    const result = getEffectivelyEnabledItems(config);
+    
+    // Test O(1) lookup performance
+    const startTime = process.hrtime.bigint();
+    const hasItem = result.prompts.has('playwright-generate-test');
+    const endTime = process.hrtime.bigint();
+    
+    assert(hasItem === true, 'Should find enabled item');
+    // This is more of a structural test - Sets provide O(1) lookups by design
+    assert(result.prompts.size > 0, 'Should have enabled prompts');
+  });
+
   console.log(`\nTest Results: ${passedTests}/${totalTests} passed`);
   
   if (passedTests === totalTests) {