Window of Twilight Portal Effect

there seems to be an optimization issue with the "Window of Twilight Portal Effect"

it drastically lowers the fps when they're closed vs when they're open
in my map-device room in Beacon of Salvation:
- no portals at all 130-150fps
- 6 portals but 3 to 4 open 90-100fps
- all 6 portals closed 50-55fps

the funny thing is so when some of them are open, there is a lot more light and shadow to be calculated unlike when all of them are closed. yet the fps drastically decreases when they're not open.

i'd love to use that effect since it suits the whole theme, but the numbers above are done after removing any decor in the room.
adding some fire, pets or what ever further adds to the issue of fps drop, although not by much surprising enough.

pls look into this. thx <3

edit: having standard portal effect but a room full of fire (10+), pets, statue, maraketh soldiers and more decor, i still have some 120-140fps depending on which corner i'm at. the south-east cliff does seem to drop the fps too, but nowhere near as bad as the Twilight Portals.

edit2: my graphics are set to potato. maybe the portals lack a low-gfx variance, which on top of some wonky stuff results in this drastic fps-drop?