Calling the Thecus N5550 a NAS is almost selling it short, because it's much more than that. It's a network server for at home or in the office, and you can use it to download files, as a digital photo album, and for connecting IP cameras. You can connect it to a computer obviously, but also tablets and smartphones let you access it via apps.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips