I think playing faster isn't spelled out correctly. In 9 ball, it really shouldn't take that long to process what the correct shot is. It's this processing that goes haywire for so many of the slow players. It's like they're not following the same simple pattern recognition practices that everyone else is. So they wander around looking at random angles multiple times, and cue ball paths that aren't even legitimate options. Then, there's the players that stay down on the ball forever, like a John Morra. I just don't get how this is helpful in the long run.
I think the trick is to think calmly and sometimes even slowly, but then transition to a quicker execution phase.
I do think most slower players don't actually like playing slow, but they've just had bad results when they speed up so they're just sort of stuck. I also think players like JB who start out later in life tend to play slower as they take a more regimented approach to the game. They miss out on the just playing to have fun phase of the game.
I totally agree that there are players who think things out too much, but there are far more who don't think things out enough at times when they should have. And contrary to what some say, over thinking rarely causes much issue, whereas under thinking causes issues often.
John Morra has the best fundamentals in pool in my opinion although several others are right there as well. You say you don't get how staying down that long is helpful, but I don't see how it couldn't be if he is doing it right (to fully clear his mind and commit to the shot among other things), and I really don't see how it could be hurtful. He isn't being ridiculous about it. He is reducing his errors over what they would be if he went faster, same as you would do if you slowed down a little, although I don't think his pace is the exact right pace for everybody, it varies a little.
As far as Jennifer or anyone else playing slower because they started later in life, and that they don't actually like playing slow, and that they missed out on the "fun phase" of the game, well that all sounds kind of dumb to me and I don't see any evidence for any of it and it defies common sense if you give it any thought at all.
I don't think the thinking, or the execution, phases either one should be fast as it just induces errors that didn't have to be there. They don't have to be stupid slow either as that doesn't serve a purpose once it gets past a certain point. There are very few pros who shoot fast which should tell people something. Efren? Slow, careful, calculated. SVB? Slow, careful, calculated (not stupid slow, but very careful slow). Archer same thing. Same with most pros aside from a few like Filler, Shaw, and Woodward, all of whom now play better than they used to because they have slowed down some, and all of whom would play better yet if they slowed down even more.
I think you think you wouldn't enjoy pool as much if you slowed down, and you end up mixing that up in your mind as being that you wouldn't shoot as well if you slowed down, maybe subconsciously conflating the two to make sure you never feel like you have to give it a fair shot. I haven't ever even seen you play but statistically speaking, and based on the things you've said, there is almost no doubt you would play better if you slowed down a bit. The only question is how much would get you to that perfect sweet spot, but for the vast majority that sweet spot is at a slower pace, at least by a little bit, than that which they currently play.