I think I may have found the reason for that: Each corpse that is on your map will add 1 icon when boarding the boat. I noticed when I raided someone with a boat and each kill I got, the icon list started to increasing. Which also decreased my FPS alot. I had to use RuntimeGC to remove the corpses, after that the icon list was normal again and my FPS got a lot better. This also includes the corpses in your homebase btw.