Once upon a time, the land of Far Harbor was shrouded in a patchy, low framerate fog? Bethesda recommends that users who have experienced performance issues with Fallout 4 Far Harbor re-download the DLC.

The issue was particularly prevalent with consoles, especially the PS4. The framerates would actually drop to 15fps in some of the worst

Source: Hot Hardware – Fallout 4 Far Harbor Performance Issues On PS4 Can Be Fixed By Re-Downloading DLC