Looking a flows across an interface for a 3 month period is a lot of data. You could increase the report time by asking for a shorter time period,. Also running you drives in RAID 0 performs much better than RAID1.
Not sure I'd run my DB on RAID 0 but I do have many other transactional databases running on MSSQL running on lesser machines that have much better performance. Websense is a great example I have one location where the Websense DB is >12GB and report generation happens in seconds and there are many more records/transactions in that DB to search and report on than the NetPerfMon DB.
I'd have to say I've pointed out our hardware setup only to communicate that hardware is not the problem here and really this looks like some type of an indexing and/or application issue that may benefit from some tweaks.
Do you have any documents on configuring/managing the NetPerfMon database for best performance? A 10GB SQL database on this hardware should not be having these long delays when pulling a report.