Используйте вариантный массив, чтобы ограничить количество обращений vba к рабочим листам:
Sub Loop_Test()
Dim sht1 As Worksheet
Set sht1 = Worksheets("Sheet1")
Dim fcst As Worksheet
Set fcst = Worksheets("Sheet2")
Dim lr As Long
lr = fcst.Cells(Rows.Count, "A").End(xlUp).Row
Dim dta As Variant
dta = fcst.Range(fcst.Cells(4, "D"), fcst.Cells(lr, "O")).Value
Dim otpt As Variant
ReDim otpt(1 To UBound(dta, 1), 1 To 1)
With sht1
Dim i As Long
For i = LBound(dta, 1) To UBound(dta, 1)
otpt(i, 1) = Application.Sum(Application.Index(dta, i, 0))
Next i
.Range("P4").Resize(UBound(dta, 1), 1).Value = otpt
End With
End Sub
Редактировать
СУММА (ИНДЕКС ()) медленная, быстрее добавить отдельные части.
Sub Loop_Test()
Dim sht1 As Worksheet
Set sht1 = Worksheets("Sheet1")
Dim fcst As Worksheet
Set fcst = Worksheets("Sheet2")
Dim lr As Long
lr = fcst.Cells(Rows.Count, "A").End(xlUp).Row
Dim dta As Variant
dta = fcst.Range(fcst.Cells(4, "D"), fcst.Cells(lr, "O")).Value
Dim otpt As Variant
ReDim otpt(1 To UBound(dta, 1), 1 To 1)
With sht1
Dim i As Long
For i = LBound(dta, 1) To UBound(dta, 1)
Dim j as Long
For j = lbound(dta,2) to ubound(dta,2)
otpt(i, 1) = otpt(i, 1) + dta(i, j)
Next j
Next i
.Range("P4").Resize(UBound(dta, 1), 1).Value = otpt
End With
End Sub
Проверено на 50000 строк и результат был почти мгновенным.