Is your transfer server-side or client-side. Are there time-consuming operations that you perform while transferring the items? Or just the integrated transfer is used? In which browser do you notice the slow-downs? In general, there used to a slow down (event browser halt) in IE when a listbox was bound to a significant number of items (for example more than 1000). However, we have worked a lot in that direction and there is significant improvement when the latest version is used. An important note here is that this applies only to the client-side transfer. The server-side transfer does not have problem with the amount of items, it is the connection speed that matters.
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