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):