Loading Message when Page Loads

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Threading;

public partial class AnotherSlowPage : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btnPageLoad_Click(object sender, EventArgs e)
    {
        //Some operation that takes long time
        Thread.Sleep(5000);
        hdnLoaded.Value = "true"; //Setting a hidden variable will help avoiding a loop of postback triggered from client side script.
        divLoading.Visible = false; // Make the loading message invisible
        lblMessage.Visible = true;
    }
}