"rows to the value of $500 were inserted into the spreadsheet but somebody forgot to press F9 in order to recalculate the spreadsheet formulae."
Why would you have turned off auto mode ? I rarely did even when I had huge sheets. Recalculate was pretty quick.
Even on this old 1.6GHz Celeron laptop using LibreOffice I've just altered the first value in a set of 400000 sine calculations each dependent on the another and it took < 1sec