i've faced this issue before and i'm facing the same issue again. I've a treeview in which one of its nodes NodeA is having 1000 child nodes under it.
I've set the CheckChildNodes property to true for the treeview. So when i check or uncheck the node NodeA, i'm getting the specified script warning message (but once in 50 times i'm not getting the message and the check/uncheck works fine).
Locally i've tested with 1500 nodes and it is running okay. But i've lot of other controls in the page in the original application.
I've tried the solutions given in this link - http://www.telerik.com/help/aspnet-ajax/troubleshooting-treeview-script-causes-ie-run-slowly.html
and none of them worked.
Can you provide any insights on this?