What works? Filling down as I described should be quite fast, converting to a Table will allow the formulas to self populate as rows are added.
Switching calculation to manual might speed things up.
The Filling down worked, I converted the range to a table and then applied manually the filling down it took around 6 minutes, then I applied the filling down by vba and it also took 6 minutes, I just have tryied also to change the calculation to manual and at the end of the code a switched again to automatic and it took 15 minutes,
The range or table has 50,000 rows I usually don´t add rows to this table but I have to recalculate the formula since other values in the worbook change, so what I do after I recalculate the formula I turn to values the formula range to avoid slowing down the worbook.