I cannot pin-point the exact effect that is causing it, but it is caused by looking in a particular direction (north from the entrance).
For example, when you zone in the frame drop will already be in effect as you are facing north. If you turn completely to the west, there is a 20-30 fps gain.
It’s worth mentioning I never noticed it being this bad until the latest valor patch. With everything set to 1 or disabled in the graphics settings, I am dipping to 15-20 fps. My hardware isn’t super solid, but I was able to tank this dungeon reliably until the patch. Sadly, I can no longer participate in this dungeon.
If you could please look into this it would be much appreciate as some nice Guardian loot drops there