>>722
Sub sample1()
Dim i Range
Range("B1") = Range("A1")
For i = 2 To 6
Cells(i, 2) = Cells(i - 1, 2) + Cells(i, 1)
Next i
end sub

A列が不特定多数で出力の速さを求めるのなら

Sub sample2()
Dim i Range,j Range
j = WorksheetFunction.Count(Range("A:A"))
Application.ScreenUpdating = False
Range("B1") = Range("A1")
For i = 2 To j
Cells(i, 2) = Cells(i - 1, 2) + Cells(i, 1)
Next i
Application.ScreenUpdating = True
end sub

間違えた

Sub sample1()
Dim i As Long
Range("B1") = Range("A1")
For i = 2 To 6
Cells(i, 2) = Cells(i - 1, 2) + Cells(i, 1)
Next i
end sub

A列が不特定多数で出力の速さを求めるのなら

Sub sample2()
Dim i As Long,j As Long
j = WorksheetFunction.Count(Range("A:A"))
Application.ScreenUpdating = False
Range("B1") = Range("A1")
For i = 2 To j
Cells(i, 2) = Cells(i - 1, 2) + Cells(i, 1)
Next i
Application.ScreenUpdating = True
end sub