APFS is just for your main system SSD/HDD and other media you specifically format as such. It should not have any bearing on the MiniMags or their performance. If it’s a recent conversion to APFS on your system, things do go slow with all the initial indexing and conversion. I’ve seen some performance issues like this if a file system is converted to APFS instead of just a fresh format and reinstall of everything. If you’re copying between the MiniMags and other external media, APFS should not be a factor at all.

Is Activity Monitor showing anything else gobbling up your system performance? How are other things running? It may not be the MiniMag or RED reader hardware, it could be an issue with the other external media. How is the other media connected? And what is it? With the upgrade to the new OS, you could be running into more indexing issues on the external media, especially if it’s a large drive or RAID that already has lots of files on it.