The transfer speed for my devices on the latest version have mysteriously dropped to ~2KB per second when transferring via data disk. I have noticed it has been slowly getting.. slower, but now it is very slow. On latest macOS and the only change I have made was pulling out the USB cable on one that was in data mode without doing anything to eject properly. However, another device I have I did not do that too, and it is equally slow. I have a third, totally clean device, I can attempt next. After that - will try a different OS.
Alright - so I think the culprit is when you have pdx that contain lots of little files. It tends to move slowly on average for those, but bigger files transfer quite fast in the same pdx. so overall average of the transfer time goes down. But with lots of subsequent small files - very slow.
Thanks for the info, a good lesson in performance and optimisation to not use many small files. Noted!