The RadComboBox supports virtualization, so you can implement it in order to speed up the opening. You can find more information in this
Hope this helps. Please let us know if you have further questions.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!