I can suggest you to implement an ItemTemplate and load the data with the Load On Demand mechanism. Thus you could implement the checkboxes in the item template and enhance the speed performance. I have prepared a sample project for you, demonstrating the suggested approach.
However please note, that using more than a thousand checkboxes in a RadComboBox control, could be quite unfriendly for the end users.
The range of the selection is too large and they will have to scroll a lot between items in order to find which one to check.
That is why using checkboxes in case there are so many items to choose from is not recommended from usability point of view.
Please find the sample attached.
All the best,
the Telerik team
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