To reduce worksheet interaction and speed up the macro:

Sub M_snb()
  sn = usedrange.columns(4).offset(11).specialcells(2).resize(,3)

  For j=1 to ubound(sn)
     if sn(j,1)=5 then sn(j,1)=sn(j,3)
  Next

  cells(5,12).resize(ubound(sn))=sn
End Sub