Other people have already talked about why you are having performance issues with the Pi. As for a better NAS solution you will probably be better off with a used desktop PC from the last 10 years. If the computer doesn’t have enough SATA ports you can get a sata addon card or HBA (host buss adapter) addon card flashed in IT mode. You should be able to find a lot of options on eBay. Maybe people can chime in with specific models to look at.