By default, a VirtualizingStackPanel creates an item container for each visible item and discards it when it is no longer needed (such as when the item is scrolled out of view). When an ItemsControl contains a many items, the process of creating and discarding item containers can negatively affect performance. When VirtualizationMode is set to Recycling, the VirtualizingStackPanel reuses item containers instead of creating a new one each time.
When VirtualizingStackPanel cannot recycle item containers, it uses the standard mode of virtualization, which is to create and discard item containers for each item. The following list describes cases when the VirtualizingStackPanel cannot recycle item containers:
The ItemsControl contains item containers of different types. For example, a Menu might have items that use both MenuItem and Separator objects as the item containers.
You explicitly create the item containers for the ItemsControl.