Both RadListBox and ListBox controls derive from ItemsControl and share the common grouping functionality. The ItemsControl is the base class which has build-in integration with CollectionViewSource and both controls are slow because the ItemsControl is slow when grouped.
I could suggest using TreeListView control if possible. The control's documentation ca be found here - http://www.telerik.com/help/wpf/radtreelistview-overview.html
Hope this helps.