docs(faq): add question about localhost being slow if no IPv6

Fix #6784

Author: vkarpov15

docs/faq.jade

@@ -352,10 +352,10 @@ block content
     doc.createdAt = new Date(2011, 5, 1).setHours(4);
     doc.save(); // Works
     ```
-    
+
     <hr id="parallel_saves" />
-    
-    **Q**. Why does calling `save()` multiple times on the same document in parallel only let 
+
+    **Q**. Why does calling `save()` multiple times on the same document in parallel only let
     the first save call succeed and return ParallelSaveErrors for the rest?
 
     **A**. Due to the asynchronous nature of validation and middleware in general, calling
@@ -366,11 +366,25 @@ block content
 
     **Q**. Why is **any** 12 character string successfully cast to an ObjectId?
 
-    **A**. Technically, any 12 character string is a valid [ObjectId](https://docs.mongodb.com/manual/reference/bson-types/#objectid). 
+    **A**. Technically, any 12 character string is a valid [ObjectId](https://docs.mongodb.com/manual/reference/bson-types/#objectid).
     Consider using a regex like `/^[a-f0-9]{24}$/` to test whether a string is exactly 24 hex characters.
 
+    <hr id="slow-localhost" />
+
+    **Q**. I'm connecting to `localhost` and it takes me nearly 1 second to connect. How do I fix this?
+
+    **A**. The underlying MongoDB driver defaults to looking for IPv6 addresses, so the most likely cause is that your `localhost` DNS mapping isn't configured to handle IPv6. Use `127.0.0.1` instead of `localhost` or use the `family` option as shown in the [connection docs](https://mongoosejs.com/docs/connections.html#options).
+
+    ```javascript
+    // One alternative is to bypass 'localhost'
+    mongoose.connect('mongodb://127.0.0.1:27017/test');
+    // Another option is to specify the `family` option, which tells the
+    // MongoDB driver to only look for IPv4 addresses rather than IPv6 first.
+    mongoose.connect('mongodb://localhost:27017/test', { family: 4 });
+    ```
+
     <hr id="add_something" />
-    
+
     **Something to add?**
 
     If you'd like to contribute to this page, please