As I am not entirely sure how you implement your scenario, I cannot tell for sure what is causing the issue. However, Well, the delay caused by the generation of the bitmap is the most probable cause.
If this is indeed the case you can try calling the UpdateLayout() method of the DragCue in the DragInfo() event handler or you can implicitly set size to the writable bitmap used as a DragCue.
Give this approach a try and let me know if it works for you. However, if this doesn't help, if you can send us a sample project illustrating the issue we will be able to further investigate it.
All the best,
the Telerik team
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