Report SSRS takes time to generate

I am generating SSRS reports in a ReportViewer in ASP.NET with VB.NET, I have not been able to solve that the first time the report is generated takes too much time. How is that delay solved?

I add Code as a charge to the ReportViewer Report:

 
Protected Sub GenerarReporte()

        ' Asigna valores de SSRS a ReportViewer

        rvOferta.ServerReport.ReportServerUrl = New Uri("http://server/ReportServer")

        Select Case fnPedidosLeerDatos(Session("id")).Rows.Item(0)("IdTipo")

            Case 2 To 5

                rvOferta.ServerReport.ReportPath = "/Reportes/OfertaL"

            Case Else

                rvOferta.ServerReport.ReportPath = "/Reportes/Oferta"

        End Select

        'Parametros para el reporte SSRS

        Dim parametros As List(Of ReportParameter) = New List(Of ReportParameter)()

        parametros.Add(New ReportParameter("Id", Session("Id").ToString))

        'Envia los parametros para el reporte

        rvOferta.ServerReport.SetParameters(parametros)

        'Actualiza ReportViewer

        rvOferta.ServerReport.Refresh()

        'Muestra Panel q contiene la oferta

        ModalOferta.Show()

End Sub