title: Mongodb id on bulk insert performance
body: <p>I have a class/object that have a guid and i want to use that field as the _id object when it is saved to Mongodb. Is it possible to use other value instead of the ObjectId?</p>
  
  <p>Is there any performance consideration when doing bulk insert when there is an _id field? Is _id an index? If i set the _id to different field, would it slow down the bulk insert? I'm inserting about 10 million records.</p>