You are NOT virtualizing. When you set the ItemsPanel of the ListBox,
you remove the ability to virtualize.
<ListBox x:Name="Recent" Margin="8"
<ListBox.ItemsPanel> <!-- This is removing
<toolkit:WrapPanel Orientation="Horizontal" />
You can test this by removing the ItemsPanel customization and adding
1000 items. Scroll the listbox and look at the load time of the control.
Add the customization back in and you'll notice decreased performance and
increased load time.
That said, you may want to see how some of the WPF
virtualizing wrap panels work on WP8. I tried them in a WPF project I
did previously and didn't have as much luck as I would have liked, but your
results may vary.