Faster Way to Substract 2 Columns?

Perhaps

Sub TestPCI()
Dim WkRg As Range
Dim WkTb1, WkTb2
Dim LastRow As Long
Dim I  As Long

    LastRow = Range("M" & Rows.Count).End(xlUp).Row
    Set WkRg = Range("M2:M" & LastRow)
    WkTb1 = WkRg
    Set WkRg = Range("O2:O" & LastRow)
    WkTb2 = WkRg
    For I = 1 To UBound(WkTb1)
        If ((WkTb1(I, 1) <> 0) And (WkTb2(I, 1) <> 0)) Then WkTb1(I, 1) = WkTb1(I, 1) - WkTb2(I, 1)
    Next
    Range("M2").Resize(UBound(WkTb1, 1), 1) = WkTb1

End Sub