This is an ongoing issue for nearly the whole lifespan of the game now, but it feels like it got even worse in BfA.
Look at the pre expansion event: Each time one of the rares spawn there are like 100 players around and it gets impossible to actually use abilities simply because of lag (lag, not fps issues, don't mix that up). Everyone is just running on place without actually moving, the rares teleport sometimes from place to place and bag and then they die without actually doing anything.
Like this just feels SO bad. Either fundamentally improve the structure of the game so that events like this can be handled or create those events in another way (Scenarios perhaps?)
Its a bit unfair, because the game is really old, but during those events you really FEEL how old it is.
Edit: For everybody who still does not understand what is going on, I have created the following video:
As you can see, my FPS are fairly ok (around 50-60 FPS at this place, even with the big number of players around as well the fact that I am recording my screen at the same time).
You will also see that my FPS stay this way for almost the whole fight, but when you look at my ability bar you will see how my abilities get pressed but don't actually get exectured. You can see other players talk about this exact issue in /say in German during the fight.
Then suddenly close to the end my whole screen freezes. This was new, I never had this before and it was not something I initially talked about, but also here you can see players talk about it right after the kill is done in /s.
After checking with around 20 of them, it seems like every single player on this kill was affected by this, not just me.
Also Asmongold stream capture about this (doesnt matter if you like him or not, this is about the technical issue):
And to point this out again, this is not an "this event only", as e.g. Nazjatar PVP events and Korraks Revenge are also affected by this.