title: MongoDB Query taking too long for a single document
body: <p>This question might duplicate, but I have not found an answer on SO or even Googling for it.</p>
  
  <p>I have a quite simple mongodb database, where I store <code>agents</code> that are simply defined by <code>{__id, username, __system}</code> and some other properties (nothing too wild, and almost no nesting). I have created both indexes for <code>__id</code> and <code>username</code> (note the double underscore on __id).</p>
  
  <p>Right now, I have a single docuemnt on <code>agents</code>, and querying <code>{username: 'jo', __system: 'test_system'}</code> takes 210ms on average.</p>
  
  <p>I can't find the problem since a single document should be way faster to read and indexes are present on the db.</p>
  
  <p>Thanks a lot!</p>