Thank you for the sample code provided.
I created a test WebPart project and tried to reproduce the issue - but without success.
On my side RadComboBox opens without delay - please take a look at the demonstration video
that I have recorded.
I am attaching here the sample - please download it, add the necessary references, deploy it and give it a try.
Can you modify it in order to reproduce the issue and send it back to us?
Thank you in advance.
the Telerik team
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