Is there anything (horror) that you do have pacing issues with? I think it'd be interesting to see if I felt differently about what you feel is slow. That would be a hard one to disagree on. I think pacing is a very obvious function.
I'm struggling to think of an example specifically from horror. I think when I made the comment, it was usually experiences with dramas where things are done at an intentionally slower speed and historically were known for a longer runtime (now everything's long, apparently) often result in an experience where the pacing can feel like a challenge in the sense that one often isn't in the mood for them when they're tired. i.e. Not something you'd start late at night. e.g. I often try to watch 2001 on New Year's Eve, trying to end it around midnight. I've seen it enough times, it practically flies by. But as a teen encountering it for the first five or six times, I would have conked out during the On the Beautiful Blue Danube soundtrack (I know because I did).
That's always the experience that jumps to my mind when issues of pacing in serious-minded movies comes up.