Unlike the other platforms, the Far Harbor DLC for Fallout 4 did not run like a dream on the PS4. Luckily VG247 reports that a fix is here (at least for the U.S....sorry Europe). Players who have noticed significant performance issues while playing their DLC should take the following steps to re-download the whole thing...

• Confirm Fallout 4 is not running. Highlight Fallout 4 in your PS4 dashboard, press OPTIONS, and select CLOSE APPLICATION to ensure Fallout 4 is not running minimized.
• Select RELATED ITEMS. You’ll find this under the Fallout 4 icon in the PS4 dashboard.
• Select MY ADD-ONS.
• Navigate to the DOWNLOAD ARROW next to Far Harbor and select it.
• You should see a notice that Far Harbor has been Added to Downloads.
• Wait for the downloadable content to finish downloading and installing before starting Fallout 4.

Sounds easy enough! Good on Bethesda for being quick to jump on the problem and patch it so quickly!