MagickPistacho

Thanks JorgeB, I'll try to install iotop and see how it works.
But lo-and-behold, I think I found the culprit just before going to bed yesterday: it seems there was an unsolicited scrubbing process going on my HDD, and it was taking forever. Stopping it seemed to do the trick, I'll see if it continues to behave by the time being.

Now I just need to figure out how to delete old datasets occupying precious space that for some reason ZFS isn't letting me to destroy, and why one or twice a week a duplicate of the "media" share but named "Media", with capital M, appears suddenly.

If the issue with the HDD doesn't revert back in a couple days, I'll come back and mark this one as solved.