Hi Stanley,
USB would be a lot slower than other disks, and that includes e-SATA enclosures, local drives, SAN drives (obviously).
Speed also depends on the types of files being backed up. Lots of small files would mean inconsistent spinning on the platters of your HDD, while backing up a database/Exchange for instance would see a more consisten speed.
In this case, your bottleneck is backing up to that USB drive, and there is no way to speed that up.
If you can look at DAS, or an external drive with an e-SATA connection, that would see some improvement.
Laters!