diff --git a/lib/array-set.js b/lib/array-set.js
index fbd5c81c..3bf62379 100644
--- a/lib/array-set.js
+++ b/lib/array-set.js
@@ -44,22 +44,35 @@ ArraySet.prototype.size = function ArraySet_size() {
 /**
  * Add the given string to this set.
  *
+ * Returns the index of the string within the set: the existing index for a
+ * duplicate insert, or the new index for a fresh one. Hot generator/consumer
+ * paths use this return value to avoid a follow-up `indexOf` call (one Map
+ * lookup per mapping for source and one for name).
+ *
  * @param String aStr
  */
 ArraySet.prototype.add = function ArraySet_add(aStr, aAllowDuplicates) {
-  var sStr = hasNativeMap ? aStr : util.toSetString(aStr);
-  var isDuplicate = hasNativeMap ? this.has(aStr) : has.call(this._set, sStr);
-  var idx = this._array.length;
-  if (!isDuplicate || aAllowDuplicates) {
+  if (hasNativeMap) {
+    var existing = this._set.get(aStr);
+    if (existing !== undefined) {
+      if (aAllowDuplicates) this._array.push(aStr);
+      return existing;
+    }
+    var idx = this._array.length;
     this._array.push(aStr);
+    this._set.set(aStr, idx);
+    return idx;
   }
-  if (!isDuplicate) {
-    if (hasNativeMap) {
-      this._set.set(aStr, idx);
-    } else {
-      this._set[sStr] = idx;
-    }
+  var sStr = util.toSetString(aStr);
+  if (has.call(this._set, sStr)) {
+    var existingIdx = this._set[sStr];
+    if (aAllowDuplicates) this._array.push(aStr);
+    return existingIdx;
   }
+  var newIdx = this._array.length;
+  this._array.push(aStr);
+  this._set[sStr] = newIdx;
+  return newIdx;
 };
 
 /**
diff --git a/lib/source-map-consumer.js b/lib/source-map-consumer.js
index f31964c0..a7cabc86 100644
--- a/lib/source-map-consumer.js
+++ b/lib/source-map-consumer.js
@@ -1445,14 +1445,11 @@ IndexedSourceMapConsumer.prototype._parseMappings =
         if(source !== null) {
           source = util.computeSourceURL(section.consumer.sourceRoot, source, this._sourceMapURL);
         }
-        this._sources.add(source);
-        source = this._sources.indexOf(source);
+        source = this._sources.add(source);
 
         var name = null;
         if (mapping.name) {
-          name = section.consumer._names.at(mapping.name);
-          this._names.add(name);
-          name = this._names.indexOf(name);
+          name = this._names.add(section.consumer._names.at(mapping.name));
         }
 
         // The mappings coming from the consumer for the section have
diff --git a/lib/source-map-generator.js b/lib/source-map-generator.js
index 5d8f6d0a..fa506418 100644
--- a/lib/source-map-generator.js
+++ b/lib/source-map-generator.js
@@ -128,19 +128,16 @@ SourceMapGenerator.prototype.addMapping =
     }
 
     // Resolve source/name to integer indices once here so MappingList.add
-    // can store them as i32s in the slab. ArraySet.add is idempotent (its
-    // own has() check internally), and indexOf is a fast Map.get afterward.
+    // can store them as i32s in the slab. ArraySet.add returns the index
+    // (existing or new), so this is one Map op per source/name instead of
+    // the add+indexOf pair.
     var srcIdx = -1;
     if (source != null) {
-      source = String(source);
-      this._sources.add(source);
-      srcIdx = this._sources.indexOf(source);
+      srcIdx = this._sources.add(String(source));
     }
     var nameIdx = -1;
     if (name != null) {
-      name = String(name);
-      this._names.add(name);
-      nameIdx = this._names.indexOf(name);
+      nameIdx = this._names.add(String(name));
     }
 
     // _validateMapping enforces numeric original.line/column when original is
@@ -279,16 +276,8 @@ SourceMapGenerator.prototype.applySourceMap =
         }
       }
 
-      var newSrcIdx = -1;
-      if (source != null) {
-        newSources.add(source);
-        newSrcIdx = newSources.indexOf(source);
-      }
-      var newNameIdx = -1;
-      if (name != null) {
-        newNames.add(name);
-        newNameIdx = newNames.indexOf(name);
-      }
+      var newSrcIdx = source == null ? -1 : newSources.add(source);
+      var newNameIdx = name == null ? -1 : newNames.add(name);
 
       newMappings.add(genLine, genCol, origLine, origCol, newSrcIdx, newNameIdx);
     }