I never played BB on anything besides the base PS4 and it ran pretty choppy in quite a few areas. The Slim is more or less the same as the OG performance wise based on what I've been told, and I've read it runs better on the Pro but I can't imagine how much better it could possibly be.
Despite the hardware shortcomings the game is enjoyable the entire way through. I don't recall ever hearing about a dev crunch period leading to incomplete/cut areas like Izalith and the sixth archstone. What's interesting about BB is how there are semi-optional bosses (what that means will become apparent at some point if you pay attention to what precisely triggers parts of the map to open up) and how few bosses are required to clear the game. Most of the content is off the beaten path so definitely explore and early at that. As with all of these types of games, if you're playing blind, it's worth it to keep a little notepad handy to keep track of yet unexplored routes, unexplained bullshit, and general filters.