Related posts
Note: I remember there were more posts (one with screenshots) but I can’t find them - yet. 
I suspected ships, trees, pillars, and other objects making up the ruins on the island. And I checked if there were any unnecessary objects under the map. Then I even narrowed the view range (FOV) to more closely observe the changes in performance. I saw drops in FPS even when the camera was not aimed at a specific set of objects.
I began to look especially at the vegetation. Drawing fog on textures with transparency is costly. 
I noticed that the ground in the distance has a few bright spots.
Without camera movement, the bright spots changed color, which meant that they were not only rendered but also animated. Then I checked where the grass was appearing and how the GPU was behaving to make sure. I have found no other places (besides The Isle of Dawn) where this light-colored grass appears.
I searched for a long time for the cause of the drop in productivity in this area. I hope that after the corrections everything will run smoothly - if not, then I will keep looking. 