Duple Backup very slow USB backup F4-423

[PatientSteve]

I have an F4-423 with a 9Tb TRAID BTRFS setup of 2 x 4Tb and 2 x 3Tb 3.5" HDDs, this is connected by the original supplied USB cable to a D5-300 which has 3 x 3Tb and 2 x 2Tb 3.5" HDDs as a JBOD formatted as BTRFS.
I am using newly installed Duple Backup to do a Hybrid backup scheduled to run monthly. The backup of the first folder from the NAS (my Movies folder) which is 6.5Tb in size has completed in very nearly six days. I set it to compress the files which I would assume would slow it down, but this seems very slow indeed. When checking up on the process the copy rate seemed to hover around 10 - 12Mb only.
It's now backing up my TV Shows folder. I assume the next months backup will be incremental only so will hopefully not do another six day copy.
Can anyone help or is this about right?
Thanks.