The length of the show has nothing to do with it. Sure if it was 26 eps of the exact style they had it would've been torture, but it being 12 or so doesn't matter. It's pacing isn't even moderate no matter who you are. You may be the kind of person who can find the violence alone so interesting that it doesn't matter, but that alone won't hold me.
also Gunslinger Girl is The Flash in comparison, and it's amazing too lol. But I agree that it's start wasn't the fastest thing around, but still amazing in comparison to this.