Is there a way to optimize this method?

        ' Equivalent of the AT() function in VFP
        ' expC1 Search for
        ' expC2 Search string
        ' expN1 Occurence
        ' expN2 Mode
        Public Function At(ByVal tcSearchFor As String, ByVal tcSearchIn As String, _
         ByVal tnOccurence As Integer, ByVal tnMode As Integer) As Integer
            Dim lnCounter As Integer = 0
            Dim lnLocation As Integer = 0
            Dim lnOccured As Integer = 0

            ' In this case we actually have to locate the occurence
            If tnMode = 1 Then
                lnLocation = 0
            Else
                lnLocation = tcSearchIn.Length
            End If

            ' Loop through the string and get the position of the required occurence
            For lnCounter = 1 To tnOccurence

                If tnMode = 1 Then
                    lnLocation = tcSearchIn.IndexOf(tcSearchFor, lnLocation)
                Else
                    lnLocation = tcSearchIn.LastIndexOf(tcSearchFor, lnLocation)
                End If

                If lnLocation < 0 Then
                    Exit For
                Else

                    ' Increment the occured counter based on the current mode we are in
                    lnOccured = lnOccured + 1

                    ' Check if this is the occurence we are looking for
                    If lnOccured = tnOccurence Then
                        Return lnLocation + 1
                    Else
                        If tnMode = 1 Then
                            lnLocation = lnLocation + 1
                        Else
                            lnLocation = lnLocation - 1
                        End If
                    End If

                End If

            Next

            Return 0
        End Function

The reason I ask is that I thought if using a StringBuilder would be better here. This is considering that I have some string of up to 20k to be passed here. so, if tcSearchFor is 20k, could this be a factor?