https://www.polygon.com/2018/5/18/17368696/dark-souls-remastered-blighttown-ps4-pro

Blighttown is an area of the original Dark Souls that combined a high degree of difficulty in combat and navigating the environment with a punishingly low frame rate. It’s one of the game’s challenges that often felt unfair, due to the performance issues hobbling your ability to move and fight with the same ease found in other parts of the game.

And it looks like it was fixed in Dark Souls: Remastered.