Two things:
1.) There are some issues arising on PS4 right now as the devs continue cracking on the wider issues. Stay tuned for updates and stuff. Some of this may simply vanish.
2.) You're probably reaching the limit of what the game can reasonably do for length. (Dark Souls / an MMO this is not.) The game takes about 200 hours to completely play through the main quest and most side quests for both standard game and NG+ combined. It starts to have definitive issues after the ~1,600 hour mark, but will probably have
some issues long before that. I would estimate that anything after 400 hours is beginning to push the engine into places it was not designed to go.
Hopefully you're able to make it through main quest, at least. If not, now might be a great time to start considering a fresh playthrough.
Separate topic: are you experiencing any performance issues? You're on PS4 or PS4 Pro?