I had once found a fast method to backup device partitions to a computer using
pv (provided by a TWRP temporary boot image).
pv ran on the Android device and was called using
adb, specifically, I think, using
cat might have been involved.
dd was not used, I'm 85% sure of that, because I need/needed to see the copy progress.
I am unable to find it now. What was it?