One thing I noticed recently is that if I look straight down, or straight up at the sky my FPS drops to 18. That is with a 1080GTX, 5930k CPU and obviously an SSD. That is with a 3000m view distance from center of Stratis island. It has the same effect as changing your view distance to 7000m+ and going to corner of map and putting the whole island in perspective.
My only theory is that looking straight up or straight down bring the entire island into perspective on the XY grid. So the game uses the player 3d camera to get it's XYZ perspective, but only uses XY to filter objects for processing. So this is why looking down to loot corpses can get especially laggy.
I custom modified my CHVD to do a bunch of things different, still experimental. Firstly, when player is in 3rd person force their view distance down to 2000m. Second, when looking down at corpse drop view distance to 1000m, makes looting easier if you were lagging. Third, bounding box check to determine if player is in building, if so then drop view distance to 1000. One player on my server boasted over 80fps when in store buildings.
My changes are still experimental though, and there are still some odd quirks I am working out. And of course this does nothing for server FPS, only the client.