You’ll get the most benefit from pre-aggregating not for a specific query but in a general way. Sum orders group by user in a view and then use triggers to update that view. That way to get the orders for a given user it’s a simple lookup and all the calculation is done. And this way you can build up meta reports from these general aggregates.
Just to be super clear don’t use the triggers to rebuild the view every time but merely to add one to the sum’d account when a row matches — I wasn’t sure I made that clear. Good luck! And do put up a blog post or something if it works out