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.
Looks like you're zeroing the disk. Hard drives are faster on the outside of the platters compared to the inside because there's more rotational velocity on the outside. Since the data is moving directly across the disk as its writing, yes, you'll see a speed difference during this process.
See https://superuser.com/questions/643013/are-partitions-to-the-inner-outer-edge-significantly-faster
Also related :)
Yup I am zeroing it.
That explains the declining speed. Thanks a lot for the insight!