More of a comment than a question.
So I've been dealing with the ultra slow re-balancing performance and I've given up. I have a USB DAS populated with fairly large drives and when attempting to re-balance the throughput is so slow that it is worse than watching paint dry. I'm not sure if this is because it is a DAS, but it is unaccapetably slow, we're talking read/write speeds of a couple MB/s. I followed the instructions of editing the setting.jron file, and have tried increasing the priority. No improvment.
The workaround is to do this manually. I identify the file(s) that I want to move from one drive to the other, and use swap space (storage that is not part of the drivepool). I cut/paste the file from the pool to the swap space. I then modify/create file placement rules so that when the file is placed back in the pool it goes to the correct drive. File placement rules are quite easy and straight forward and my workaround works much faster that letting drivepool re-balance.