Purging will not help. If you add 15,000 order line items per day, and you have 20 business days in a month, to see a month-end report you need to add up fields in 300,000 records. It doesn't matter if those are the only 300,000 records in the table, or if the table contains the whole 3,600,000 from the last year or 10,800,000 from the last 3 years. It takes a long time to add up 300,000 records, period. It might take as much as one whole minute, depending on your hardware. If it's also taking a long time to isolate which 300,000 records to process, what you need is better indexing, not purging.

If your user needs instant reports, you can use an OLAP approach. Basically, as you save each transaction you also add the amounts to a set of running totals, so the processing time is divided into miniscule slivers and added to each transaction, but the report is as simple as printing out the numbers.