gridView 显示“正在加载中，请稍后.....”

我在网上看到别人是用UpdatePanel实现，代码如下：
<asp:ScriptManager ID="ScriptManager2" runat="server">
                        </asp:ScriptManager>
                        <asp:UpdatePanel ID="UpdatePanel2" runat="server">
                            <Triggers>
                                <asp:AsyncPostBackTrigger ControlID="btn_statistics" EventName="Click" />
                            </Triggers>
                            <ContentTemplate>
                                <asp:GridView >
                                  此处省略
                                </asp:GridView>
                            </ContentTemplate>
                        </asp:UpdatePanel>
                        <asp:UpdateProgress ID="UpdateProgress2" runat="server">
                            <ProgressTemplate>
                                <span style="font-size: 12px">正在加载数据，请稍后......</span>
                            </ProgressTemplate>
                        </asp:UpdateProgress>
然后在后台：
protected void btn_statistics_Click(object sender, EventArgs e)
{
System.Threading.Thread.Sleep(5000);
}
但是，我有一个问题，这样的话，当5秒中后，“正在加载数据，请稍后......”这些字就没有了，怎么实现当gridview数据绑定完了之后，才消失“正在加载数据，请稍后......”这些字？？？