Trying to speed up GUI response time - Combobox SelectIndexValue event slows down app

[bajanpoet]

I realize that the response time of my GUI is extremely slow! I have three combo boxes - I want the user to select a template from one drop down list, which will populate the second drop down list with customers related to that template. When a customer is selected, the third drop down list is to be populated with items related to that customer.

Each code snippet that selects the next level (template -> customer; customer -> item) is stored in a separate procedure:

Private Sub SelectTemplate()
        'User selects the template desired from the drop down list
        Try
            Select Case cboTemplate.Text
                Case Is = "South"
                    SQL_Text = "SELECT itemno, [item desc], barcode, cat, baseunit, stockunit FROM SBI_SOUTH_CUST_ITEMS"
            End Select
            RegionString = SQL_Text
        Catch ex As Exception
            MessageBox.Show("Error:" & ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End Try
    End Sub

Private Sub PopulateCustList()
        'Shows list of customers in Customers drop down list
        Dim SelectCust As String
        Try
            SelectCust = "SELECT * FROM OTM_SOUTH_CUSTOMERS"
            Dim SQLCom As New SqlCommand(SelectCust, SQLCon)
            da = New SqlDataAdapter(SelectCust, SQLCon)
            da.Fill(Custds, "cust")

            CustomerLoading = True 'Set flag on
            cboCustomer.DataSource = Custds.Tables("cust")
            cboCustomer.DisplayMember = "name"
            CustomerLoading = False 'Set flag off

        Catch ex As Exception
            MessageBox.Show("Error:" & ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End Try

Private Sub ShowCustomerDetails()
        'Show items details for specific customers in Items drop down list
        Dim da As SqlDataAdapter
        Dim dt As New DataSet

        Try
            Dim SQLCom As New SqlCommand(SQL_Text, SQLCon)
            da = New SqlDataAdapter(SQL_Text, SQLCon)
            da.Fill(dt, "details")
            dgView.DataSource = dt.Tables("details")
            dgView.AutoResizeColumns()
            ItemLoading = True
            cboItem.DataSource = dt.Tables(0)
            cboItem.DisplayMember = "item desc"
            ItemLoading = False
        Catch ex As Exception
            MessageBox.Show("Error:" & ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End Try
    End Sub

Those procedures are called from the SelectIndexChanged events of cboTemplate and cboCustomer, respectfully.

I find that in my testing, these SelectIndexChanged events are fired numerous times - every time I add a DataSource, every time I add a DisplayMember.

I need to code this logic in such a way that the GUI does not hang for 5 seconds each time a user makes a change. What can I use other than SelectIndexChanged?

(One way I tried to get around this is the boolean CustomerLoading and ItemLoading. I change them to True when the datasource and displaymember are being loaded and false otherwise, and try to skip the myriad times the procedures are fired by testing for a False value in the SelectIndexChanged, but either I'm not doing it right, or I'm missing something.)

Please assist!