@p45cal;

Not using Excel functions in VBA seems to be always faster.

Since you sum all values per 'row' you could try to reduce the code to:

If colm > 1 Then 
    With Application 
        mysum = .Sum(.Index(resultsAry, rw)) 
    End With 
End If
Basically:

Sub M_snb()
  sn = Sheet1.Cells(1).CurrentRegion
  
  For j = 1 To UBound(sn)
     MsgBox Application.Sum(Application.Index(sn, j))
  Next
End Sub