diff --git a/lodash.js b/lodash.js
index ffe815dd7..bdfdc6039 100644
--- a/lodash.js
+++ b/lodash.js
@@ -976,9 +976,6 @@
    * // => [1, 2, 3, [[4]]];
    */
   function flatten(array, shallow) {
-    if (shallow) {
-      return concat.apply(ArrayProto, array);
-    }
     var value,
         index = -1,
         length = array.length,
@@ -987,7 +984,7 @@
     while (++index < length) {
       value = array[index];
       if (isArray(value)) {
-        push.apply(result, flatten(value));
+        push.apply(result, shallow ? value : flatten(value));
       } else {
         result.push(value);
       }
diff --git a/perf/index.html b/perf/index.html
index bb731ce44..52adf326c 100644
--- a/perf/index.html
+++ b/perf/index.html
@@ -12,11 +12,11 @@
 		</style>
 	</head>
 	<body>
-		<script src="../lodash.min.js"></script>
+		<script src="../lodash.js"></script>
 		<script>
 			var lodash = _.noConflict();
 		</script>
-		<script src="../vendor/underscore/underscore-min.js"></script>
+		<script src="../vendor/underscore/underscore.js"></script>
 		<script src="../vendor/benchmark.js/benchmark.js"></script>
 		<script src="../vendor/firebug-lite/src/firebug-lite-debug.js"></script>
 		<script>
diff --git a/perf/perf.js b/perf/perf.js
index c07fa1801..d365d515d 100644
--- a/perf/perf.js
+++ b/perf/perf.js
@@ -48,7 +48,12 @@
           _ = window._,
           lodash = window.lodash,
           numbers = [],
-          object = {};
+          object = {},
+          words = [
+            'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine',
+            'ten', 'eleven', 'twelve', 'thirteen', 'fourteen', 'fifteen', 'sixteen',
+            'seventeen', 'eighteen', 'nineteen', 'twenty'
+          ];
 
       for (var index = 0; index < 20; index++) {
         numbers[index] = index;
@@ -119,8 +124,6 @@
       })
   );
 
-  /*--------------------------------------------------------------------------*/
-
   suites.push(
     Benchmark.Suite('each object')
       .add('Lo-Dash', function() {
@@ -139,6 +142,50 @@
 
   /*--------------------------------------------------------------------------*/
 
+  suites.push(
+    Benchmark.Suite('flatten deep')
+      .add('Lo-Dash', function() {
+        lodash.flatten([1, [2], [3, [[4]]]]);
+      })
+      .add('Underscore', function() {
+        _.flatten([1, [2], [3, [[4]]]]);
+      })
+  );
+
+  suites.push(
+    Benchmark.Suite('flatten shallow')
+      .add('Lo-Dash', function() {
+        lodash.flatten([1, [2], [3, [[4]]]], true);
+      })
+      .add('Underscore', function() {
+        _.flatten([1, [2], [3, [[4]]]], true);
+      })
+  );
+
+  /*--------------------------------------------------------------------------*/
+
+  suites.push(
+    Benchmark.Suite('groupBy with callback')
+      .add('Lo-Dash', function() {
+        lodash.groupBy(numbers, function(num) { return Math.floor(num); });
+      })
+      .add('Underscore', function() {
+        _.groupBy(numbers, function(num) { return Math.floor(num); });
+      })
+  );
+
+  suites.push(
+    Benchmark.Suite('groupBy with property name')
+      .add('Lo-Dash', function() {
+        lodash.groupBy(words, 'length');
+      })
+      .add('Underscore', function() {
+        _.groupBy(words, 'length');
+      })
+  );
+
+  /*--------------------------------------------------------------------------*/
+
   suites.push(
     Benchmark.Suite('keys')
       .add('Lo-Dash', function() {
