Finally found the thread and person where I read about this. 
It works btw.
EDIT 26th July: Did it in seasonal.
When you are in front of the cave you change background FPS to 1.
Click cave then instantly alt+tab for 20 seconds.
other than that nothing else matters (very likely).
If it doesn’t work you can try again and also alt+tab right after you click on the npc until they scream (and also stick to lower side of screen from when you load in the cave until NPC click) but I don’t think this changes anything.
It’s probably not about latency but about how assets are loaded. The lag thing might work (see below, well…above) as well, for the same reasons, but I’d try this first.
Just ‘fixed’ this (and it’s unlikely to be luck).
So:
Don’t abandon, don’t clear event (very likely irrelevant), go to the cave, change game to windowed (not fullscreen window), lock cutscene fps, set max fps to 59, set min fps to ONE, uncheck the “lock cursor to game window” so you can minimize (or Windows+M which I forgot about), accept…click to enter cave (1st time in that game after char select), right after you click the entrance minimize the game and do something else for a few seconds, alt tab back, hug the lower wall until you are under the NPC, click her ONCE and without doing anything else (no moving, nothing) quickly minimize game again and don’t click on it again until they scream.
If it doesn’t work walk outside the cave and log out and I think you have to wait out the 9s timer if you want to relog and still be next to the game. Or was it the other way around (exit right away and relog within 8s)?
Also, don’t be in combat when you enter the cave (which might be why clearing events helps).
I did clear Raze the Effigies event in the attempt that worked (2nd one where I tried to minimize with 1 fps background cap - I don’t think it made a difference, it’s about how it loads assets…although ppl had problems with necro pets that auto-aggro on the bugged barrier…but if it’s spawned bugged already you are probably done for anyway in that game).
Proof: https://www.twitch.tv/videos/1862482560