VBA function too slow

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MS-Excel

The following VBA function calculates very slowly in Excel. I am interested in finding someone who can provide an add-in for this in C. I am willing to pay for this service.

Function DRXL( A, B, Row)
'A and B are integers and row is the worksheet row number'

    Set Lows = Range("Lows")
    Set Highs = Range("Highs")
    Set TR = Range("TR")
    HH = 0
    For n = A To B
        Value1 = (Highs.Cells(Row) - Lows.Cells(Row - n)) / Sqr(n)
        If Value1 > HH Then
            HH = Value1
        End If
    Next
    DRXL = HH / Application.Average(Range(TR.Cells(Row), TR.Cells(Row - 20)))
End Function
Thanks,
Peter Day