<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
	<ContentTemplate>
		<asp:Panel ID="PnlWait" runat="server">
			Bitte warten, hier wird gearbeitet...
		</asp:Panel>
		<asp:Panel ID="PnlMain" runat="server" Visible="false">
			Arbeit erledigt!
		</asp:Panel>
		<asp:Timer ID="TmrStartup" runat="server" Interval="100" />
	</ContentTemplate>
</asp:UpdatePanel>

Protected Sub TmrStartup_Tick(ByVal sender As Object, ByVal e As EventArgs) _
  Handles TmrStartup.Tick
	'Zeitintensiven Vorgang simulieren
	Threading.Thread.Sleep(5000)
	TmrStartup.Enabled = False
	PnlWait.Visible = False
	PnlMain.Visible = True
End Sub