Hello Annie,
Thank you for the clarification.
The Expandable List control uses javascript in order to expand and collapse items. If you have a very large list and use this mode the page will get a bit heavier on javascript and loose some performance. We have tested the control with displaying close to 4000 items and it worked as expected although experiencing some slow down. Attached is a video of how the control works with many items in IE 7. The same behavior is observed in IE 6.
Please inspect your page if you are loading any other scripts which are throwing JS errors. These errors might prevent the proper working of the control. Also, do you have any customizations over the cotnrol?
Sincerely yours,
Radoslav Georgiev
the Telerik team
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