A postback is always required by the RadFilter control because every time you add a new filter expression it is persisted on the server and later used to construct the actual filter query.
You can try to wrap the RadFilter inside an UpdatePanel or ajaxify the whole content of the RadWindow so that the RadFilter inside it performs asynchronous postback and the grid is not refreshed every time. This help article explains how you can do this: http://www.telerik.com/help/aspnet-ajax/radwindow-ajaxifying.html
All the best,
the Telerik team
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