If you are installing via NSP, ensure the DLC files match the Region ID of your base game. Mixing an EU base game with US DLC NSPs will result in the extra content failing to load. Performance Tips for the Best Experience
If you are managing your own file setup (e.g., using NSP/XCI files), ensuring "extra quality" typically means integrating all updates to benefit from performance stability. assassins creed iv black flag switch nsp dlc extra quality