I think you're misunderstanding what the USB ports on a NAS do. Those are to connect an external HDD to the NAS for backing up files from the NAS onto the external drive. You generally can't connect a computer to the USB ports of a NAS at all. To do so would be like plugging two computers into each other's USB ports. It'll probably cause a short in most cases since both are supplying voltage.
The only exception to this I know of is Drobo (which I really wouldn't recommend to anyone). Drobo does allow direct USB connection as well as a sort of NAS type iSCSI functionality. But, it's not going to have all the other nice features a good NAS like a Synology or Qnap would have.
Perhaps a better option than trying to use something as DAS and NAS is to just add DAS to the computer where you need the high speed, then synchronize between the NAS and DAS on the fly. That way it'll seem fast on the machine where you need the speed, but everything will still be getting synced back over time.
We deal in several Tb of data that we move around daily as we're a data recovery company dealing in full disk images. I've got a massive amount of DAS storage in my workstation (two RAID 6 arrays actually), but the critical data which needs to be backed up and shared interoffice gets two-way synced to a Synology NAS via their Synology Drive software. This way I never see any bottleneck due to network speed limitations but the backups and sync are still getting done.