Convert formula to a worksheet function

[sharky12345]

I'm using this to populate a column but it takes ages to run as there are over 15000 rows - is it possible to turn this into a worksheetfunction and if yes I assume it will run a lot quicker?

Range("S2").Formula = "=IFERROR(INDEX(DATA!F2:F" & LastDataRow & ",AGGREGATE(14,6,(ROW(DATA!D2:D" & LastDataRow & ")-ROW(DATA!D2)+1)/(DATA!C2:C" & LastDataRow & "=A2)/(DATA!D2:D" & LastDataRow & "<G2),1)),"""")"
Range("S2").AutoFill Destination:=Range("S2:S" & LastRow), Type:=xlFillDefault
Application.Calculation = xlCalculationAutomatic
Range("S2:S" & LastRow).Value = Range("S2:S" & LastRow).Value

 

[Legacy 143009]

What does this formula do?
If you explain it in detail, then maybe a faster vba code can be an option.

 

[Peter_SSs]

1. In the active sheet, can the same value appear more than once in column A?
   
   
2. Are you sure that there shouldn't be some $ signs in those formulas that you are inserting into the workbook? I would have thought that all the multi-row ranges would have their rows locked.
   
   
3. 15,000 rows in which sheet? Or both? If not both then please specify approximate number of rows in each one