If someone's goal is to mass-downvote all comments made by someone else, this wouldn't stop them at all. Just keep the page open in another tab, and every N minutes downvote the next comment.
For people who read quickly and vote in real time, it would be inconvenient. (For example, Stack Exchange has a similar thing: I can read a short comment in 1 second, but I am allowed to vote only once per 5 seconds. So I read 20 comments in a row and then close the page without voting, because I don't see a point in waiting.)
Given that the aim is to prevent mass downvoting of a single person, it could perhaps be more usefully implemented as a timer that prevents one person from downvoting the same user more than X times every N minutes (for some low X, perhaps only one or two).
This would have less effect on the casual browser (since, normally, you'd be reading comments from several people) while still making it harder for the mass downvoter.
There may need to be some changes to the database to implement this, of course.
To whoever has for the last several days been downvoting ~10 of my old comments per day:
It is possible that your intention is to discourage me from commenting on Less Wrong.
The actual effect is the reverse. My comments still end up positive on average, and I am therefore motivated to post more of them in order to compensate for the steady karma drain you are causing.
If you are mass-downvoting other people, the effect on some of them is probably the same.
To the LW admins, if any are reading:
Look, can we really not do anything about this behaviour? It's childish and stupid, and it makes the karma system less useful (e.g., for comment-sorting), and it gives bad actors a disproportionate influence on Less Wrong. It seems like there are lots of obvious things that would go some way towards helping, many of which have been discussed in past threads about this.
Failing that, can we at least agree that it's bad behaviour and that it would be good in principle to stop it or make it more visible and/or inconvenient?
Failing that, can we at least have an official statement from an LW administrator that mass-downvoting is not considered an undesirable behaviour here? I really hope this isn't the opinion of the LW admins, but as the topic has been discussed from time to time with never any admin response I've been thinking it increasingly likely that it is. If so, let's at least be honest about it.
To anyone else reading this:
If you should happen to notice that a sizeable fraction of my comments are at -1, this is probably why. (Though of course I may just have posted a bunch of silly things. I expect it happens from time to time.)
My apologies for cluttering up Discussion with this. (But not very many apologies; this sort of mass-downvoting seems to me to be one of the more toxic phenomena on Less Wrong, and I retain some small hope that eventually something may be done about it.)