[dnanian]

It's a little slower -- obviously, the sparse image has to be stored somewhere, so it's going to go as slow as copying to that would, with a bit of additional overhead.

If you want to be able to reboot from the backup, though, you should definitely clone to a volume, not an image. Images can't be booted from (though they can once restored), sparse or not.

You may be having a large number of unusual retries on the drive. You might want to check its smart status in Disk Utility, to make sure you don't have an imminent failure situation. Also, when you reformat, zero the drive to re-map as many sectors as you can before restoring your data.