You have probably already noticed that, apart from the ability to add a custom layout, a collection view is very similar to a table view. When you look at what collection view does under the hood to maintain excellent scrolling performance, you will find even more similarities. The collection view is optimized to show as many cells on screen as quickly as it possibly can while keeping its memory footprint as small as possible. These optimizations are important for table views but they are even more important for collection views because a collection view might show a lot more cells on screen at a time than a table view does. The following diagram visualizes this:
The fact that collection views show so many cells at once has made it so that Apple added an extra optimization to it compared to table views. A table view always loads one or two items that are near the user's current scroll position. This is fine, especially considering that table views only one new...