I’ve been using Apple Numbers to help a professor friend of mine with grade calculations. It turns out that if you import a sheet from Excel, it can often get into a state where the performance becomes unacceptable - despite the actual functionality appearing to be fine.

At one point, entering a new number into a sheet with a single table of around 2000 numeric cells and no formulae at all was taking more than 5 seconds.

Exporting the data to CSV and re-importing it cured this problem.