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

dist/lodash.underscore.js

@@ -1594,15 +1594,20 @@
    * // => { 'name': 'fred' }
    */
   function omit(object) {
-    var indexOf = getIndexOf(),
-        props = baseFlatten(arguments, true, false, 1),
+    var props = [];
+    forIn(object, function(value, key) {
+      props.push(key);
+    });
+    props = difference(props, baseFlatten(arguments, true, false, 1));
+
+    var index = -1,
+        length = props.length,
         result = {};
 
-    forIn(object, function(value, key) {
-      if (indexOf(props, key) < 0) {
-        result[key] = value;
-      }
-    });
+    while (++index < length) {
+      var key = props[index];
+      result[key] = object[key];
+    }
     return result;
   }
 
@@ -1667,9 +1672,9 @@
         result = {};
 
     while (++index < length) {
-      var prop = props[index];
-      if (prop in object) {
-        result[prop] = object[prop];
+      var key = props[index];
+      if (key in object) {
+        result[key] = object[key];
       }
     }
     return result;