Can I ask — why do you need all the leaf cells? If we want to know total gross profit for the year — why can't we compute that as "total revenue - total COGS", rather than aggregating every net profit split by product / region / month?
In any case, if you sum a list of numbers you have to look at each value. There’s no way around it. You can parallelize, like MapReduce, but the number of items is unchanged. You can preaggregate, but at some point you need partial sums.