I'm going to start by saying that I don't know how this works under the hood - all I can do is speculate based on how things appear. Based on the existence of this thread, we can stipulate that enemies spend way too much time making up their minds about what to do. This is especially painful in large battles. It would probably help if there were a fast way to narrow the decision space for each enemy.
What if, at the beginning of each combat round, every non-player-controlled creature in the fight chooses one enemy to focus on: probably whoever is closest to them, but possibly whoever they were focusing on from the previous round or an alarm that they want to trigger or whoever is the biggest threat to them. What kinds of things a specific character cares about could be set by Larian in advance, (e.g. if I'm the queen's elite guard, I only care about enemies that get within 60' of her, but if I'm a big hulking brute, I'm going to look for another brute to square off with). Whatever the method for this choice is, it should be simple and fast. Then, when it comes time for each creature's turn, all it does is consider the ways to attack whatever they chose to focus on (instead of considering every way to get at each potential target).
This method certainly isn't flawless. It will potentially create other issues, e.g. "What do I do if the target I chose at the beginning of the round is dead or out of reach when my turn comes up?" But I would think, in combination with some other fixes, that it could speed things up quite a bit.
