The calculation for a top level on a large list cannot be split so as the list grows, the calculation becomes increasingly inefficient. Consider if you really need the total. If you need to have the totals, look to add intermediate parent “totals” to make the calculations more efficient, of use SUM to aggregate for validations
Best Practice article:
Top Level Item and Parent Hierarchy
Questions on this topic? See 1.05-07 Avoid Top Level for large flat lists in our Planual forum.