When in WebService binding RadScheduler pre-renders the tables for all views on the server and sends them on the client. Then it populates the Appointments through the web service. Since the tables are rendered on the server, you have to postback in order to change the grouping, as it involves changing the table layout.
What I can suggest is to put the RadScheduler inside a RadAjaxPanel or an UpdatePanel, which will improve the performance in this scenario.
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