The problem is that one partner can have hundreds of thousands of orders per month. And if we'll fetch the full order object it will be bad for performance. can we fetch only the specific "amount_total" field without rest of data ? or alternatively there is a query that makes this sum calculation ?

amount_total is the field that saves the net amount of partner orders. so you have to find the summation of this field after fetching orders in between date range. for the date, you can query either with the create_date of record or order date [date_order].