I agree that the described scenario hits RadBreadcrumb's
performance badly, but it is expected because of the control's design.
I suggested using of a RadTreeView
for loading large amounts of data, but unfortunately it can't mimic RadBreadcrumb's
In order to implement your scenario easily you can try to edit the RadBreadcrumb's
template and replace the existing StackPanel with a VirtualizingStackPanel.
All the best,
the Telerik team