I'm running into the same issue. I understand that balancing might not be designed to be super fast, but there's a difference between being "not fast" and having a read/write speed of maybe half the nominal speed of the drive, and being so painfully slow that it's literally ~75x slower than the nominal read/write speed of the drives.
In my case, I have one full 12 TB HDD in my pool and added another 12 TB HDD recently. When I test transfers from one to the other without DrivePool (just plain old Windows Explorer copy), I get speeds of 100-200 MB/s. However when DrivePool tries to do the balancing, the Windows Task Manager shows me write speeds of 2 MB/s on the new drive... It seems to me that this is probably a bug that should be looked into.
Because of this I am forced to disable the balancing otherwise it's going to take 4 months...
Please consider looking into this.