title: performance issue for updating mongo documents
body: <p>I have a mongo collections with about 4M records, 2 of my document fields of this collection are date as string, and I need to change them to ISODate, so arote this small script to do that:</p>
  
  <pre><code>db.vendors.find().forEach(function(el){
      el.lastEdited = new Date(el.lastEdited);
      el.creationDate = new Date(el.creationDate)
      db.vendors.save(el)
  })
  </code></pre>
  
  <p>but it takes foreverrrr...and I added indexes to those fields, can't I do this in some other way which will be reasonable time to complete?</p>