It certainly looks to be tied to asset streaming, at least in the case with ACT 3. Ran into this myself twice during ACT 2, but here's an example of it happening with the Kuo-toa;
Though this isn't just an issue with asset streaming because the game does this from time to time even during regular gameplay, such as when attacking someone and the attack taking several seconds to actually trigger. This delay can also be easily reproduced when doing actions which affect multiple things at once, such as the Illithid Repulsor ability.
I even managed to reproduce it easily in ACT 1 on purpose to the point it entirely bricks the player's input;
Ultimately Larian has a very nasty bug lying somewhere in the engine or some mechanic, because I've never seen a singleplayer game acting like an online game and introducing delayed actions as if I'm playing online. I'm absolutely sure they're aware of the issues with ACT 3 though and this was not intended whatsoever. If anything a fix will most likely be coming within the next two weeks.
