Sub function1()
Dim i As Long
For i = 1 To 1000000
Cells(i, 1) = "A" & i
Cells(i, 2) = "B" & i
Next i
End Sub

Sub function2()
Dim j As Long
For j = 1 To 1000000
Cells(j, 3) = "C" & j
Cells(j, 4) = "D" & j
Next j
End Sub