Same issue here. Tried everything, from completely removing addons to reinstalling WOW. This bug is only in GAMBIT portals, every other loading screen in the game is fine.
Have seen many people reporting the same bug and yet to hear an answer or fix from Blizzard. It’s been months…
I feel like “loading screen” might be the wrong description here. Some people mentioned being able to use spells on other people in their party while still having the white screen blinding them. I’ve found that I’m actually able to move and cast spells.
I’m basically “loaded” into the area. But the white screen lingers on for past when you’re loaded. Oribos, where the stakes are zero so it’s not as much a topic, also has the same issue when using the teleporters.
It’s too bad there isn’t just some box we could check or command we could type to tell the game not to use teleporter visual effects on our screen.
i can cast instant cast spells in it, but anything channeled, it won’t let me…at least last i checked. so as a healer it’s really deadly, i always give a heads up.
it’s been consistent with me though - my first run always gives me a longer load/whatever screen after the first boss…and as long as i start the dungeon from stormheim’s entrance, the first load thing is fine. 
I too experience this. I have no clue, and it’s just this dungeon. It’s after the first teleport like you explained takes forever, so I could move just couldn’t see. We didn’t 2 chest it because of the lost time they waited for me on both portals.
Same goes for me for about several weeks. Can’t get rid of this stupid VFX portal and it ruins 10-20 seconds of the run. THANKS OBAMA.
I’m just commenting here to sub to this thread. I don’t want to do a reinstall of windows, so here I am waiting for a suitable solution that goes beyond playing this dungeon more than once a day.
I found the solution! You need WoW installed on an SSD versus HDD. I have 3 hard drives in my computer and moved WoW to the SSD and it resolved the white screen issue in Gambit.
I don’t have a single HDD in my tower. My C: drive is an m.2 drive, and wow is installed on another SSD drive.