I have plenty of RAM and nothing really running in the background. Volumetric Fog is turned off. Even turned down the land graphics and no change.
I've attacked other harbors, with several ships, and it doesn't do this.
This lag/fps drop is similar to what happens when I'm approaching Pearl Harbor. But unlike Pearl the fps doesn't go back up when I move away from the area. Just weird that it's only happened like this when I attack Truk.
Maybe some new type of Japanese weapon?