Hi
As the contents of the panel which is collapsed are very large,you can delay by several seconds to load the content.
You don't need to load it when the whole page load.when the whole page is loading,set the panel unvisible.
Then load the content of the panel.
Code as follow:
<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
protected void Button1_Click(object sender, EventArgs e)
{
System.Threading.Thread.Sleep(3000);
TextBox1.Text = DateTime.Now.ToString();
}
</script>
<script type="text/javascript">
function delayLoad()
{
document.getElementById("<%= Button1.ClientID %>").click();
document.getElementById("<%= UpdateProgress1.ClientID %>").style.display = "block";
}
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
<title>Untitled Page</title>
</head>
<body onload="setTimeout('delayLoad()',3000)">
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<div visible="true"><asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" /></div>
</ContentTemplate>
</asp:UpdatePanel>
<asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
<ProgressTemplate>
Waiting..............
</ProgressTemplate>
</asp:UpdateProgress>
</div>
</form>
</body>
</html>
More informatin,see:Delaying Content Load using Timer and UpdatePanel
Thanks