Your link to the time system in BG 1 and 2 is good, but it is a bit incomplete in the technical sense. For instance, how many attacks can a character have per round? The answer used to be five, if starting from the absolute beginning of a round. But if a few frames are wasted then the character won't be in time for the first attack and would not do their full round's worth of attacks until the following turn. How many off hand attacks? Only one, always. Use a +1 attack weapon off hand and you get another main hand attack.
Except BG2 did offer a way to break past this limit with Improved Haste and the Whirlwind HLAs. With those, you could have more than 5 APR, but only with those. Push to 5.5 APR normally and that's just wasted, but 3.5 APR + Imp Haste equals 6 attacks being executed. To my knowledge, it was done by letting the character perform two rounds at double speed during the normal round. And if you checked the log of a dual wielder during an Imp Haste round, you would find two off hand attacks. Just a funny little detail.
But the point is, I am almost certain that they fixed each combat round action to a particular frame during a round. The default was 30 frames per second, so they presumably divided the round into 180 frames and then fixed certain actions to certain frame numbers in order to get all the animation sprites to look sensible and end in time for the next round. I do not know this for a fact, of course, but I suspect it.
And this is what I mean with simplification and lack of fine control. There was very little in terms of controlling who went first or who went in what order or who took exactly what path in their movement. Combat was that lightning fast because each character's combat round was fairly simplified and all had their combat rounds at the exact same time. You either managed to be in range and time for your actions on that round or you did not and then you'd do it next round. You could pause the game, of course, but trying to maintain exact control over everything was like herding cats.
Now imagine 5E rules instead of 2E rules, meaning actions and bonus actions and free actions and reactions and turn order stuff and complex movement mechanics in the form of threat zones and disengagement and jumping distances and dashing and potion throwing and whatnot. Trying to handle that for six party members in real time (even with pause) while having to figure out what a throng of enemy critters are doing sounds like the kind of task that would make my hairline recede even further. I don't particularly like this added complexity, to be brutally honest, but it is part of 5E and I also don't think people would be happy if Larian just house ruled it away.
That being said, I absolutely agree with your criticism of how combat plays out in BG3 currently. Bigger fights are way too slow, too long, too drawn out, and enemies take way too long to get their backside going. There really needs to be some sort of fast forward until the next thing happens that actually affects a player-controlled character directly. I liked the combat of BG1 and 2 more than this. But that was 2E. I don't see how it could be done while preserving the flavour of 5E.