title: Render countdowns on each flat list item
body: <p>I'm doing an auctions app with react-native and Firebase, I'm using a <code>FlatList</code> to render each auction component.</p>
  
  <p>On each auction component want to show a countdown with the reminder time, I'm retrieving the end date of the auction from Firebase.</p>
  
  <p>But if I update the state every second, of each of my auction components my app  gets really slow.</p>
  
  <p>Is there a better way to do that?</p>