Coming into Chapter 4 on the PS5, the game is approaching unplayable for me as well. Eternal slow motion in combat, multiplayer or solo. If it has to do only with extra katakuri on your map, I'm not sure why the problem is suddenly popping up in multiplayer games as well, unless other people are also experiencing the same issues, and I am just getting that slowdown and lack of responsiveness due to the map of the host (which is not always a console player, but often a PC player).
Also, Chapter 4 is when most people start throwing down multiple shrines for resource gathering. I'm wondering if they are somehow eating up performance resources in some way.
Clearing katakuri off the maps may be a way to get performance back, but ironically, katakuri on the maps were a draw for the game. My maps are fairly free of clutter, as I never host. If katakuri are causing this slowdown all of a sudden, in my case, it would seem to be dragon katakuri causing the issue.
Whatever it is, maybe there needs to be the equivalent of a cache flush, or a button on the map to clear all basic katakuri, with further options to also clear specific dragon katakuri, if this will help. I don't knwo what it is honestly, but suddenly, after the game performing well through Chapters 1-3, its a slowdown fest in Chapter 4, and sort of miserable to play, when you combine that with the amped up speed of monsters and the need for precision timing.
Edit: I hosted a game for a couple of friends and I didn't experience slowdown. I'm back to thinking that it has to do with the karakuri in your map, as I cleaned mine up a little bit.