title: UIViewController calls sizeForItemAtIndexPath for all cells on load
body: <p>I have a UICollectionView with UICollectionViewFlowLayout. While testing with large data (5,000+ cells) I noticed a lag when loading. I found out that at loading the <code>sizeForItemAtIndexPath</code> is called for EVERY cell. Since I am doing a messaging like application and each cell is of different height, this causes height estimation to occur. I am not scrolling to the bottom or anything, just loading the view.</p>
  
  <p>I have made a small app demonstrating what I am saying (look at the console to see that it is calling <code>sizeForItemAtIndexPath</code> for all cells) <a href="https://github.com/ftheo/CollectionViewLoadingBug" rel="nofollow">https://github.com/ftheo/CollectionViewLoadingBug</a></p>
  
  <p>Any help would be appreciated. Thanks</p>