I'm not aware of anything you can do when using FormMule. Does FM update on the time trigger? If not, there might be a problem with the setup.
I'm guessing that if the calculations are taking a long time, it could be a problem. I don't remember ever seeing it but it is possible.
You could use the script and add a delay. When you open the spreadsheet, is there a long delay? I have seen spreadsheets where the number of rows has increased for some strange reason and that is making the calculations take longer.