Optimize _.omit for large arrays by leveraging _.difference which has optimizations for large arrays.

dist/lodash.js

@@ -2631,23 +2631,29 @@
      * // => { 'name': 'fred' }
      */
     function omit(object, callback, thisArg) {
-      var indexOf = getIndexOf(),
-          isFunc = typeof callback == 'function',
-          result = {};
+      var result = {};
+      if (typeof callback != 'function') {
+        var props = [];
+        forIn(object, function(value, key) {
+          props.push(key);
+        });
+        props = difference(props, baseFlatten(arguments, true, false, 1));
 
-      if (isFunc) {
-        callback = lodash.createCallback(callback, thisArg, 3);
-      } else {
-        var props = baseFlatten(arguments, true, false, 1);
-      }
-      forIn(object, function(value, key, object) {
-        if (isFunc
-              ? !callback(value, key, object)
-              : indexOf(props, key) < 0
-            ) {
-          result[key] = value;
+        var index = -1,
+            length = props.length;
+
+        while (++index < length) {
+          var key = props[index];
+          result[key] = object[key];
         }
-      });
+      } else {
+        callback = lodash.createCallback(callback, thisArg, 3);
+        forIn(object, function(value, key, object) {
+          if (!callback(value, key, object)) {
+            result[key] = value;
+          }
+        });
+      }
       return result;
     }