Started with 50 MBps, went down to 20 MBps shortly after and is declining slowly since. Running for 7+ hours.
HDD is 5 years old, rare use but very well kept.

Edit: external 1.5 TB HDD connected over USB 3. Overwriting with zeroes while formatting using gnome-disks.

Update:

Stopped gnome-disks ~78% and continued writing zeros using dd for the remaining sectors.
command used: #sudo dd if=/dev/zero of=/dev/sda1 bs=1M seek=1001250 status=progress (don’t copy without understanding), used seek here to skip already zeroed sectors.
write speed went up from ~14 MB/s to ~100 MB/s.

slow speed could be caused by multiple passes of overwrites by gnome-disks (not sure if it does that), or by “initializing the filesystem at the same time as zeroing” as mentioned by @ares35.

gradual speed decrement was observed in both methods, as mentioned by @Synthead.

Thanks to everyone for being so helpful.

  • Eager Eagle@lemmy.world
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    1 year ago

    Formatting is almost instantaneous, this seems to be overwriting the files for a safe deletion (a.k.a. deleting white space). In any case, it’s weird to measure a safe deletion speed as MB/s, as the operation involves multiple overwrites. I’d just format it using gparted if you don’t care about deleting the white space.

    • witchergeraltofrivia@lemmy.worldOP
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      1 year ago

      Yes I am overwriting it with zeroes while formatting (using gnome-disks). I didn’t know it did multiple overwrites, thought it did 1 pass overwrite with zeros.