Know Your Meme - Among many other things.

I'd upvote your comment, but I have to wait 8 seconds.

Oh, wait, I actually waited 10 and it still gave me the prompt about voting too fast.

I cannot tell you how viscerally disgusted I feel whenever I wait 15 seconds, upvote a comment or image I like and still get that damn prompt.

There's honestly no reason for it, it has no real purpose, it doesn't stop anyone from upvoting or downvoting everything in sight, it just makes them wait a few seconds between votes. If anything it seems counterintuitive to the website by discouraging user engagement with the site.

And unlike the fabled Edit Button, long denied to us because "This website is ancient spaghetti code upon ancient spaghetti code" this would be easier to implement because they actually wouldn't have to implement anything, they'd be un-implementing something instead.

If they got rid of the vote limiter it would be this but with cold fusion and lightspeed travel.