Probably not restic’s fault. Have a look at the specific drive being used in the WD Elements enclosure - is it an SMR drive?
These guys grind to a halt when copying many small files (which happens when restic first creates its repo). Actually, even a CMR drive will copy a few large files faster than the equivalent size in small files; it’s just the effect is greater with SMR.
This issue comes up quite regularly on the forum in relation to USB drives, and also cloud storage. There are ways you can improve the performance. Here’s the latest forum thread (this one is in relation to per-file cloud charges, but the workaround is the same):
What I didn’t say in my first post is that the USB disk was encrypted with VeraCrypt. And I remember that, in a way to quickly delete the repo, I did a quick format inside the encrypted volume of the disk with the format command of Windows. Well, maybe the filesystem didn’t like that and was altered (although no error was found).
More probably, the filesystem has gone corrupted because of the lack of free space (and the encryption didn’t help)
Anyway after a complete format of the disk then a re-creation of an encrypted VeraCrypt volume, the backup times has returned to normal. 10 hours to backup the 3 To!