Could someone please explain the significant difference in speed between a firefox updatepanel async postback and one performed in IE?

Average Firefox Postback Time For 500 objects: 1.183 Second

Average IE Postback Time For 500 objects: 0.295 Seconds

Using firebug I can see that the majority of this time in FireFox is spent on the server side. A total of 1.04 seconds.

Given this fact the only thing I can assume is causing this problem is the way that ASP.Net renders its controls between the two browsers.

Has anyone run into this problem before?

VB.Net Code

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
        GridView1.DataBind()
    End Sub

    Public Function GetStockList() As StockList
        Dim res As New StockList

        For l = 0 To 500
            Dim x As New Stock With {.Description = "test", .ID = Guid.NewGuid}
            res.Add(x)
        Next

        Return res
    End Function


    Public Class Stock
        Private m_ID As Guid
        Private m_Description As String

        Public Sub New()
        End Sub

        Public Property ID() As Guid
            Get
                Return Me.m_ID
            End Get
            Set(ByVal value As Guid)
                Me.m_ID = value
            End Set
        End Property

        Public Property Description() As String
            Get
                Return Me.m_Description
            End Get
            Set(ByVal value As String)
                Me.m_Description = value
            End Set
        End Property
    End Class

    Public Class StockList
        Inherits List(Of Stock)

    End Class

Markup

<form id="form1" runat="server">
     <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>

    <script type="text/javascript" language="Javascript">

function timestamp_class(this_current_time, this_start_time, this_end_time, this_time_difference) { 
        this.this_current_time = this_current_time;
        this.this_start_time = this_start_time;
        this.this_end_time = this_end_time;
        this.this_time_difference = this_time_difference;
        this.GetCurrentTime = GetCurrentTime;
        this.StartTiming = StartTiming;
        this.EndTiming = EndTiming;
    }

    //Get current time from date timestamp
    function GetCurrentTime() {
        var my_current_timestamp;
        my_current_timestamp = new Date();      //stamp current date & time
        return my_current_timestamp.getTime();
        }

    //Stamp current time as start time and reset display textbox
    function StartTiming() {
        this.this_start_time = GetCurrentTime();    //stamp current time
    }

    //Stamp current time as stop time, compute elapsed time difference and display in textbox
    function EndTiming() {
        this.this_end_time = GetCurrentTime();      //stamp current time
        this.this_time_difference = (this.this_end_time - this.this_start_time) / 1000; //compute elapsed time
        return this.this_time_difference;
    }

//-->
</script>
<script type="text/javascript" language="javascript">
    var time_object = new timestamp_class(0, 0, 0, 0); //create new time object and initialize it   

    Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(BeginRequestHandler);
    Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
    function BeginRequestHandler(sender, args) {
        var elem = args.get_postBackElement();
        ActivateAlertDiv('visible', 'divAsyncRequestTimer', elem.value + '');
        time_object.StartTiming();
    }
    function EndRequestHandler(sender, args) {
        ActivateAlertDiv('visible', 'divAsyncRequestTimer', '(' + time_object.EndTiming() + ' Seconds)');
    }
    function ActivateAlertDiv(visstring, elem, msg) {
        var adiv = $get(elem);
        adiv.style.visibility = visstring;
        adiv.innerHTML = msg;
    }
</script>

    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="Button1" EventName="click" />
    </Triggers>
        <ContentTemplate>
          <asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
            </asp:UpdateProgress>
            <asp:Button ID="Button1" runat="server" Text="Button" />
            <div id="divAsyncRequestTimer" style="font-size:small;">
            </div>
            <asp:GridView ID="GridView1" runat="server" DataSourceID="ObjectDataSource1" AutoGenerateColumns="False">
                <Columns>
                    <asp:BoundField DataField="ID" HeaderText="ID" SortExpression="ID" />
                    <asp:BoundField DataField="Description" HeaderText="Description" 
                        SortExpression="Description" />
                </Columns>
            </asp:GridView>

            <asp:ObjectDataSource ID="ObjectDataSource1" runat="server" 
                SelectMethod="GetStockList" TypeName="WebApplication1._Default">
            </asp:ObjectDataSource>
        </ContentTemplate>
    </asp:UpdatePanel>

    </form>