My problem was not a Slog as much as two maybe three rounds and bad guy was out. I wanted longer fights, especially, as I said earlier, run 1-3 encounters in a day so those encounters are meant to last a little longer and be a heck of a lot tougher. It was more a fix to my style of play and easy for me to do.
I often wrote up higher end encounters from scratch, creating new things to fight because the unfortunate side effect, at least in my games, is that the higher the campaign goes the more "Specialized" the enemy gets. Meaning the story/plot/ quest whatever you want to call it means most the high CR things in the monster books are not appropriate to the story/ environment etc. I reskinned some things but still only so far that stretches things.
Not a complaint or bash of the system. I know most play seems to be at the lower end and no matter how much high CR stuff they put out, chances are it still would not quite fit in my game. I don't mind the custom tailoring though I know not everyone is comfortable creating, reskinning etc monsters / bad guys etc. I am.
And I roll out in the open so hit points was always the easiest thing I could manipulate to shift the challenge. I know some people see that as a cheat but I use it both ways to easy up and to pour it one when it comes to encounter. Side effect of crafting home made stuff is occasionally screwing up in both directions Too hard, Too easy. I have to adjust for my own fallibility without making my players suffer. Too easy I usually let roll they should feel like badasses once in a while.