If "$group" is grouping on an indexed field F and if all the functions are not dependent on the rest of the document (such as $sum:1 aka count) huge improvement can be made in performance by adding '{$sort:{F:1'}} before the '{$group}'

Tested on large collection (TPCH orders denormalized with lineitems inside) about 1.5 million documents aggregating by order date (2600 different dates) all after warming the data first:

Without sort: 18-19 seconds
With sort: 2.5-2.6 seconds

On really small datasets I still see at least 25%-33% improvement with $sort so if we can do that "automatically" that would help performance.