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.
For the fastest you can get, let your drive zero itself https://archive.kernel.org/oldwiki/ata.wiki.kernel.org/index.php/ATA_Secure_Erase.html
Still may not be fast
This is not recommended for USB connected drives
I’m not sure why this page has an obsolete warning and at this point I’m too afraid to ask 😅