USB ports on a NAS are a trap (and I fell for it) ...
Most of the time your NAS is sitting on the shelf, quietly storing whatever files you send to it. However, most NASes can do more than just back up your data, especially if they have free USB ports.
Are you talking about using the USB function on the Synology? Avoid it if possible. The results will be in some cases it is faster. In others about a day later you'll wish you went over the network ...