loading rows from database to datagridview is to slow in vb.net winforms

i have a datagridview that is loading rows from a table from database and there are other columns are filled by  a calculation 

while filling the datagridview from datatable (ex.30 rows ) and then calculate the other column it takes more than 50 sec 

i don't know this delay with this small number :( 

can anyone help 

 Private Sub loadDetails(proc_id As String, proc_type As Integer)
        If OraxCon.State = ConnectionState.Open Then OraxCon.Close()
        Dim sqlstr As String = "select * from Inv_Sub where  proc_id=" & proc_id & "  and proc_type=2"
        OraxCon.Open()
        sda = New SqlDataAdapter(sqlstr, OraxCon)
        dt = New DataTable()
        sda.Fill(dt)
        inv_subDataGridView.Columns("item_name").AutoSizeMode = DataGridViewAutoSizeColumnMode.None

        If dt.Rows.Count = 0 Then
            '' isexited = False
            Return
        Else 'empty dgv

            For Each row As DataGridViewRow In inv_subDataGridView.Rows

                For Each cell As DataGridViewCell In row.Cells
                    cell.Value = ""
                Next
            Next
        End If
        If dt.Rows.Count > 0 Then
            ISLOADEDINV = True
                        Dim rows_diff As Integer = 0
            Try
                If dt.Rows.Count > inv_subDataGridView.Rows.Count Then


                    rows_diff = dt.Rows.Count - inv_subDataGridView.Rows.Count
                End If
                For j As Integer = 1 To rows_diff
                    inv_subDataGridView.Rows.Add()
                Next
            Catch
            End Try
                ReDim Preserve Unit_Perc(dt.Rows.Count)
                For i As Integer = 0 To dt.Rows.Count - 1
                    Dim cr = New Currencies(Int32.Parse(dt.Rows(i).Item("currency_from").ToString()))
                    If cr.isexited = True Then


                        inv_subDataGridView.Rows(i).Cells("id").Value = cr.id
                        inv_subDataGridView.Rows(i).Cells("temp_id_barcode").Value = cr.id

                        inv_subDataGridView.Rows(i).Cells("barcode").Value = cr.barcode.ToString()
                        inv_subDataGridView.Rows(i).Cells("item_name").Value = cr.symbol

                        inv_subDataGridView.Rows(i).Cells("unit").Value = dt.Rows(i).Item("unit").ToString()
                        inv_subDataGridView.Rows(i).Cells("unitname").Value = cr.get_unitname(Int32.Parse(dt.Rows(i).Item("unit").ToString()))
                        If dt.Rows(i).Item("val").ToString() = dt.Rows(i).Item("val1").ToString() Then
                            inv_subDataGridView.Rows(i).Cells("quantity").Value = dt.Rows(i).Item("val").ToString()
                        Else
                            Unit_Perc(i) = dt.Rows(i).Item("val") / dt.Rows(i).Item("val1")
                            inv_subDataGridView.Rows(i).Cells("quantity").Value = dt.Rows(i).Item("val1")

                        End If
                        sett.userpermission(user_IDLabel.Text, 10004)

                        If sett.costWithVat = 1 Then
                            inv_subDataGridView.Rows(i).Cells("price").Value = dt.Rows(i).Item("new_exchange_price").ToString()
                            inv_subDataGridView.Rows(i).Cells("taxval").Value = dt.Rows(i).Item("new_taxval").ToString()

                        Else

                            inv_subDataGridView.Rows(i).Cells("taxval").Value = dt.Rows(i).Item("taxval").ToString()
                            inv_subDataGridView.Rows(i).Cells("price").Value = dt.Rows(i).Item("exchange_price").ToString()
                        End If

                        inv_subDataGridView.Rows(i).Cells("discount").Value = dt.Rows(i).Item("discount").ToString()
                        inv_subDataGridView.Rows(i).Cells("taxperc").Value = dt.Rows(i).Item("taxperc").ToString()

                        Try
                            inv_subDataGridView.Rows(i).Cells("total").Value = Double.Parse(inv_subDataGridView.Rows(i).Cells("price").Value.ToString()) * Double.Parse(inv_subDataGridView.Rows(i).Cells("quantity").Value.ToString())
                            inv_subDataGridView.Rows(i).Cells("totalafterdiscount").Value = Double.Parse(inv_subDataGridView.Rows(i).Cells("total").Value.ToString()) - Double.Parse(inv_subDataGridView.Rows(i).Cells("discount").Value.ToString())
                            If sett.costWithVat = 1 Then
                                inv_subDataGridView.Rows(i).Cells("nettotal").Value = Double.Parse(inv_subDataGridView.Rows(i).Cells("totalafterdiscount").Value.ToString()) '' + Double.Parse(inv_subDataGridView.Rows(i).Cells("taxval").Value.ToString())

                            Else
                                inv_subDataGridView.Rows(i).Cells("nettotal").Value = Double.Parse(inv_subDataGridView.Rows(i).Cells("totalafterdiscount").Value.ToString()) + Double.Parse(inv_subDataGridView.Rows(i).Cells("taxval").Value.ToString())

                            End If
                        Catch
                        End Try
                        Try

                        inv_subDataGridView.CurrentCell = inv_subDataGridView.Rows(i + 1).Cells("barcode") 'temp_id_barcode
                    Catch
                        End Try

                    End If



                Next
            End If

            inv_subDataGridView.EndEdit(True)
        inv_subDataGridView.CommitEdit(DataGridViewDataErrorContexts.Commit)

        Try
            inv_subDataGridView.Columns("item_name").AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill

        Catch
        End Try
        ISLOADEDINV = False

i tried autoresizing and doublebuffer and stopdrawing but it still sooo slow 

i spent a week solving the prob and no use 

please help 

big thanx