Why backup between NAS and USB performance slow?
1. Most USB HDD in NTFS file system but under Microsoft patent, even it it workable, but NTFS/FAT USB HDD unable to get full speed on the NAS.
2. Thecus NAS default in EXT3/4 file system, if need running full speed for NAS between USBHDD, it is recommend make USB HDD in EXT3/4 file system. But note Windows unable to read EXT3/4 file system/
3. If you went to backup data in faster way, here recommend link USB HDD on your PC, and via FTP to backup data.
4. File size, due to small size file would spent more over head for file transmitting and make average speed going down, thus a 1GB file would faster than 100 files in 10MB each. Thus recommend via single huge file and able to see the performance. It is like the vehicle fuel usage, in the downtown average mileage lower the the highway. And also we could via FTP to test performance, most FTP client would have average speed log as like the FILEZILLA. https://filezilla-project.org/