The best approach to optimize
the performance speed of RadComboBox when using huge amount of items is using the load-on-demand
feature. The load-on-demand
mechanism loads the items only when the user types or clicks in the field or the drop-arrow image.
When using the load-on-demand mechanism with ItemsRequested event you should:
Set the EnableLoadOnDemand property to true.
Subscribe to the ItemsRequested event and add the items there.
Hope this helps.