For the time being you can use the following workaround:
JavaScript:
		<script>
			function fixBtnRepaint(sender, args) {
				sender._useModules = false;
			}
		</script>
ASPX:
				<telerik:RadButton ID="RadButton1" runat="server" Text="Click" OnClientLoad="fixBtnRepaint">
				</telerik:RadButton>