Image-backup takes a long time to actually start backing up

[GoTVm]

So far I've had this issue on about every system I've used the scripts on (Rpi 3-4, 1-2-8GB RAM), while writing the backup to either a NAS or a USB 2.0 or 3.0 drive.

The image-backup command starts, asks the usual info about where to store the backup image and its size, then once I confirm with y it stalls with no output for a long time, sometimes it doesn't even actually start after that. Am I doing something wrong?