I’m here to help you a bit with debugging this strange problem. Since I experience this kind of problems too.
After patch 9.0.5 release the Spires of Ascension fps drop to 15-20 affects me. For this instance I have it the whole dungeon no matter where my camera angle is, excluding when I point it straight down to the ground, then the fps bumps up to 60-90. For SoA it didnt happen before 9.0.5 in this kind of way, but the overall performance was bad in this specific dungeon before, but not that bad it was kinda playable. But I think, the problem was there before, but now it just made it worse.
Also I want to pair Empyrean Domain (new Bastion arena zone) with this problem too, before patch no problems there whatsoever and after 9.0.5 the fps problem behave the same as in SoA instance.
For SoA and ED Arena the CPU which runs at it’s highest speed otherwise, run’s there at 1,4Ghz for my instance. And it pumps so small wattage into the CPU around 3W normally 15W (+ 25W turbo boosts). There might be some power limit conflicts with GPU. (Whole time p1 or p2 limits or thermal limits the cpu doesn’t hit, it is just running at so low wattage and frequency for some strange reason.)
I also have small places where something similiar happens, it’s in ToP when you walk up the stairs into the Gorechop boss wing, on top of the stair’s is fog which causes the some behaving problem. When you pass into the new room where the trash packs are, the issue disappears. Same for DoS where is the three hallways leading to each wing of boss, you walk up the stairs from main roundabout to some boss wing and there is a small roundabout when you enter the room and there is a white portal which causes the same problem when camera is viewing through it, when you pass it more deeper into the hallway it goes back to normal.
DxDiag for help: pastebin(dot)pl/view/71118238
Seems like some problem with rendering some particle effects like fog which is in some way in SoA around the dungeon, in the arena mentioned etc.
If you need my help for any further more useful information about this problems, you can count on me and contact me.