excel fill formula till last row - excel

Question

I have below vba code to put a formula in cell AE3 then copy down to last row, i have 5000+ rows but wonder why it take so long to process (about 5 min. and still running), is there a better way to do this? i want copy down to last row as the list is dynamic data with different ranges every day. Thanks.
Sub FillRows()
Dim col_AE As String
Sheet1.Select
col_AE = "=IFERROR(INDEX(setting!C[-17],MATCH(smart!RC[-9],setting!C[-18],0)),"""")"
If col_AE <> vbNullString Then
For j = 3 To Range("A" & Rows.Count).End(xlUp).Row - 1
If Range("ae" & j).Value = vbNullString Then
Range("ae" & j).Value = col_AE
End If
Next j
End If
End Sub

Answer 1

You should turn off both ScreenUpdating and Calculations when working with large numbers of formulas.
This line If col_AE <> vbNullString Then isn't doing anything.
Option Explicit
Sub FillRows()
Application.ScreenUpdating = False
Application.Calculation = xlCalculationManual
Dim col_AE As String
With Sheet1
For j = 3 To .Range("A" & .Rows.Count).End(xlUp).Row - 1
If .Range("ae" & j).Value = vbNullString Then
.Range("ae" & j).FormulaR1C1 = "=IFERROR(INDEX(setting!C[-17],MATCH(smart!RC[-9],setting!C[-18],0)),"""")"
End If
Next j
End With
Application.Calculation = xlCalculationAutomatic
Application.ScreenUpdating = True
End Sub
The majority of the processing time is being used because the sheet is recalculating every time a formula is added. I would just turn off ScreenUpdating and Calculations and replace all the formulas. In this way I know that the formulas are consistent and that any errors introduced by users would be corrected.
Sub FillRows2()
Application.ScreenUpdating = False
Application.Calculation = xlCalculationManual
Dim col_AE As String
With Sheet1
.Range("A3", "A" & .Rows.Count).End(xlUp).FormulaR1C1 = "=IFERROR(INDEX(setting!C[-17],MATCH(smart!RC[-9],setting!C[-18],0)),"""")"
End With
Application.Calculation = xlCalculationAutomatic
Application.ScreenUpdating = True
End Sub

Answer 2

This might speed it up - turn off the screen updating while it is running.
Application.ScreenUpdating = False
Application.ScreenUpdating = True

Answer 3

Please try this:
Option Explicit
Sub fillFormula()
Dim wbk1 As Workbook
Dim lastRow As Long
Set wbk1 = ActiveWorkbook
With wbk1.Sheets("sheet1")
lastRow = .Range("A" & .Rows.Count).End(xlUp).Row
lastRow = lastRow - 1
.Range("AE3:AE" & lastRow).Formula = _
"=IFERROR(INDEX(setting!C[-17],MATCH(smart!RC[-9]," _
& "setting!C[-18],0)),"""")"
End With
End Sub