updatepane.ascx

<script type="text/javascript" language="javascript">
                Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(BeginRequestHandler);
                Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
                function BeginRequestHandler(sender, args)
                {
                     var elem = args.get_postBackElement();
                     Activate );
                }
                function Activate >            
</div>

updatepane.ascx.vb

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
            If DotNetNuke.Framework.AJAX.IsInstalled Then
                DotNetNuke.Framework.AJAX.RegisterScriptManager()
            End If          
End Sub

Protected Sub ProcessClick_Handler(ByVal sender As Object, ByVal e As System.EventArgs)
            System.Threading.Thread.Sleep(2000)
End Sub

I had the same problem after upgrading to the 4.8.0 and the easiest way that I found is turning off the Ajax in host settings.