title: Is the performance of ref.endAt().limit(n).on(...) similar for .priority and name?
body: <p>I am currently using <code>ref.endAt().limit(n).on(...)</code> to get the 'last' n values.</p>
  
  <p>All the <code>.priority</code> are null so the list is sorted by name which is a 0 padded timestamp</p>
  
  <p>It seemed that if I set the <code>.priority</code> of each item also to the timestamp that it would take more storage.  Does it?</p>
  
  <p>Regardless of whether or not it takes more storage, is there a significant performance difference for retrieving the last n sorted items if <code>.priority</code> is all null (so name sort is used) or if <code>.priority</code> are all unique and that .priority sort is used?</p>
  
  <p>I am currently designing for it to work well with 10,000 ish items in a list.  Is <code>.priority</code> or name sort better when a list gets over 1,000,000 items?</p>
  
  <p>What about using <code>ref.startAt(null, timeStart).endAt(null, timeEnd).on(...)</code>?</p>
  
  <p>I could profile, but how would I know that server load or network delays are or are not affecting it?</p>