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.
It depends, it might be doing a single pass, but doing several is quite common.
Probably selected secure erase or something which would write a bunch of random and then zero it out.
Super common in the enterprise world, probably overkill for OP.