In my experience Resource Monitor's reporting of read and write rates can lag behind what's actually happening, making it look like it's transferring more files at any given point than it really is - but that transfer graph is definitely a sign of hitting some kind of bottleneck. It's the sort of thing I'd expect to see from a large number of small files, a network drive over wireless, or a USB flash drive.
Can you tell us more about what version of DrivePool you're using (the latest "stable" release is 18.104.22.1689), what drives are involved (HDD, SSD, other), how they're hooked up (SATA, USB, other) and if you've made any changes to the Manage Pool -> Performance options (default is to have only Read striping and Real-time duplication ticked)?
Examining the Performance indicators of DrivePool (to see it, maximise the DrivePool UI and click the right-pointing triangle to the left of the word Performance) and the Performance tab of Task Manager when the bottlenecking is happening might also be useful.
Hmm. You might also want to try something other than the built-in Windows copier to see if that helps, e.g. FastCopy ?