I have about many (about 9) radcombobox placed inside RadDock in one of my appilcation's page, each radcombobox contains thousands of items.

In this case when the RadDock is expanded, it takes about a minutes to respond back.
However this problem no longer persists when I expand RadDock with the blank radcomboboxes (having no items in it).
So how to make it quick to expand with the data in my radcomboboxes?... btw I am not using any kind of transition effect while expanding.

Please advise.
Thanks in advance.

Hello Anshul,

Please, try your project by configuring RadComboBox to load on demand its contents. In case this does not solve your problem, please send us a simple running project reproducing the issue so we can work on it to provide a more appropriate solution. To be able to attach files you need to open a support ticket as demonstrated in the attached instructions.

Kind regards,
Petya
the Telerik team