Minimizing the complexity of the markup inside templates pays off in memory and in time spent on the UI thread, improving responsiveness especially while panning/scrolling.
The templates in question are the item template (see Item Template) and the control template of a List View Item or a Grid View Item (the item control template, or Item Container Style).
For an items control bound to a 1000-item collection, it would be a waste of resources to create the UI for all the items at the same time, because they can't all be displayed at the same time.
List View and Grid View (and other standard Items Control-derived controls) perform UI virtualization for you.
The concept of a viewport is critical to UI virtualization because the framework must create the elements that are likely to be shown.For example, here's markup that removes the check mark that appears by default when an item is selected, and changes the background color of the selected item to orange.There are about 25 properties with self-describing names similar to Selection Check Mark Visual Enabled and Selected Background.Restore virtualization by setting a width and height on the Items Control.Keep the number of UI elements used to render your items to a reasonable minimum.UI virtualization is the most important improvement you can make.