Bootstrap Modal Popup showing after some Delay on PostBack in ASP.Net

Hello, I'm using PostBackTrigger to get the value of the file loaded, but since the page got the return, the modal is closed, what I did was to show the modal again in the Save button event.

The problem is that there is a delay in showing the modal, this is very annoying visually, how can I solve this problem?

Thank you,

protected void grvResult_OnRowCommand(object sender, GridViewCommandEventArgs e)
{
    if (e.CommandName.Equals("cncDetail"))
    {
         System.Text.StringBuilder sb = new System.Text.StringBuilder();
  
         sb.Append(@"<script type='text/javascript'>");
         sb.Append("$('#myModal').modal('show');");
         sb.Append(@"</script>");
  
         ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "MyModal", sb.ToString(), false);
    }
}


protected void btnSubmit_Click(object sender, EventArgs e) 
{ 
     /*...more code..*/ 
     ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "openpopup();", true); 
}

<div id="myModal" class="modal fade" data-backdrop="static" data-keyboard="false" tabindex="-1" role="dialog">
    <div class="modal-dialog modal-content modal-lg">
        <div class="modal-header">
            <button type="button" class="close" data-dismiss="modal" aria-hidden="true">x</button>
            <h3>Detail</h3>
        </div>
        <asp:UpdatePanel ID="upmisticket" runat="server">
            <ContentTemplate>
                <div class="modal-body">
                    <div class="row">
                        <div class="col-md-12">
                            <div class="form-group">
  
                                <asp:FileUpload ID="idFileUpload" runat="server" />
  
                            </div>
                        </div>
                    </div>
                </div>
                <div class="modal-footer">
                    <asp:Button ID="btnSubmit" runat="server" Text="Save" OnClick="btnSubmit_Click" />
                    <button class="btn btn-info" data-dismiss="modal" aria-hidden="true">Close</button>
                </div>
            </ContentTemplate>
            <Triggers>
                <%-- <asp:AsyncPostBackTrigger ControlID="btnSubmit" EventName="Click" />--%>
                <asp:PostBackTrigger ControlID="btnSubmit" />
  
            </Triggers>
        </asp:UpdatePanel>
    </div>
</div>

<script language="javascript" type="text/javascript">
     function openpopup() {
           $("#myModal").modal('show');
     }
</script>