title: Is objectForKey slow for big NSDictionary?
body: <ol>
  <li>Assume we have very big <strong>NSDictionary</strong>, when we want to call the <strong>objectForKey</strong> method, will it make lots of operations in core to get value? Or will it point to value in the memory directly? </li>
  <li>How does it works in core?</li>
  </ol>