title: Scoping - querying nested documents performance impact?
body: <p>I want to mirror the data of an external database through their API.</p>
  
  <p>However, I also want to store a few extra fields on each document. I figured the structure would be easier to reason about, if there was a clear distinction between the data supplied by the API and the extra fields I have attached.</p>
  
  <p>Example:</p>
  
  <pre class="lang-js prettyprint-override"><code>Schema({
    youtube: {
      type: Object
    },
    'youtube.etag': {
      type: String
    },
    'youtube.commentThreadId': {
      type: String,
      index: 1
    },
    'youtube.channelId': {
      type: String,
      index: 1
    },
    'youtube.videoId': {
      type: String,
      index: 1
    },
    'youtube.canReply': {
      type: Boolean
    },
    'youtube.totalReplyCount': {
      type: Number
    },
    'youtube.isPublic': {
      type: Boolean
    },
    lastSync: {
      type: Date,
    },
    updatedAt: {
      type: Date
    },
    createdAt: {
      type: Date
    },
  });
  </code></pre>
  
  <p>In my case, scoping the API data on 'youtube' as an embedded document seemed to do the trick, but I would like to know if their is any performance hit by doing this, compares to having the fields at the outermost level of the document.</p>