I have tested the described scenario on my side with various number of Items in the RadComboBox and I was unable to reproduce the issue.
Attached you can find a simple project with ajaxified RadComboBox and RadToolTipManager in it.
I noticed in your code that the RadComboBox data bind logic is implemented in the RadAjaxManager OnAjaxRequest server event.Note that this event fires in case manual ajaxRequest from the client is evoked. When an AJAX request is triggered using ajaxRequest function from the client, the AJAX Manager itself should be set as the AJAX initiator updating the corresponding control. See the help topic below for more information on this matter:
Furthermore, as you are reporting that the RadComboBox hangs after the request, possible cause of this issue is having some js errors on the page. Therefore you can check the browser console for any errors that may cause such unusual behavior.
If you continue to experience this issue and In order for us to be of any further assistance, please open a support ticket and try to prepare a simple, fully runnable project that isolates just the problematic scenario and send it for inspection.
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