How to prevent write Paragon service from slowing down?

I'm trying to back up 600Gb over USB 2.0. If I do a file copy in Windows, I get an initial data rate of 50Mb/s and long duration average of 21Mb/s. This gives me an 8 hour overnight copy. If I use Paragon to take a full backup, I get an initial 17Mb/s, dropping to 13Mb/s when 20 mins in, 10Mb/s in an hour, and a long term rate of 5Mb/s. 26 hours in, I quit. I observe with Resource Monitor, that the Windows copy service is Normal I/O priority but the Paragon hdm... service is low I/O priority. It won't change whilst running and setting it high beforehand doesn't stick. Any ideas how to prevent Paragon write speed from tailing off?