To reproduce use the attached project. Workaround: radCheckedDropDownList1.CheckedDropDownListElement.BeginUpdate(); radCheckedDropDownList1.CheckedMember = "Selected"; radCheckedDropDownList1.DisplayMember = "Name"; radCheckedDropDownList1.DataSource = dataSource; radCheckedDropDownList1.CheckedDropDownListElement.EndUpdate();
RadCheckedDropDownList creates a visual item in the text box for each checked item. Hence, when populated with a data source with more than one thousand checked items, RadCheckedDropDownList need to create all visual items which will be displayed. This is what causes the performance issue, as creation of over 1000 visual items is heavy operation. Due to the fact, the 1000 items are not usable in this kind of control, we do not consider this as a common scenario, hence the item is declined. Still, we will constantly check the Likes of the item, and if there is considerable demand for such scenario, we will consider virtualizing the text part of the control.