1. The Ai is slow. They have enemies basically thinking too much. This makes it so they aren't so stupid, but there is too much lag and they still are stupid. If they were like other turn based games, they'd be faster and turns would flow smoother.
2. They allow each character too many crazy options. Combat should be simpler. Move, Attack and maybe Bonus Action. Next character. Same. Next. Same. The only reason it should move slowly is if you, the player, are taking your time deciding something. The more they get away from genuine D&D 5e rules, the more the AI has to think about different choices it has when moving enemies. Try Solasta. It uses turn based with 5e rules accurately, and the combat is much smoother.
3. UI is messy, so hard to find things on your turn.
There's more, but bottom line it is slow right now. It is Early Access, so Im hoping by full release the turn based system is cleaned up and faster. Dialogues and stuff too. Its all pretty messy right now.
Hi! I played Solasta, DOS games and BG3 EA and I have some programming and game design experience, not an expert though, but anyway my 2 cents on the topic:
Currently combat is indeed slow in BG3.
Main reason is indeed the AI thinking too much, however so far based on my experience with early versions of turn based games (including DOS EA), this is almost always the case until the very late phase of development so im pretty confident that this will change a lot. When devs tweak enemy behaviours, add new spells/actions etc the AI needs a rework and this usually cause bugs/lags until they do a code clean up. The trick is that its more effective and less time consuming to do the cleanup of the code at a later phase, then patching it every time something changes (that im saying based on experience haha). Its still a good thing to highlight as feedback though, as it definitely something needs to be worked on before game goes into full release.
The UI is also something im fairly confident will turn around until the full release, Larian likes iterations, so fingers crossed they can fix the current problems with it. Solasta is a great example, playing the game feels very smooth - mostly because if the intuitive and dynamic UI - compared to BG3. Also things like skills/abilities that needs double-clicking for no reason, or clicks not recognized at some times should go away with full release. There are some actions which doesn't even necessary would need an icon on the UI (looking at you jumping - which could be part of movement - and sneak attack), but we will see which direction they will take with these.
My bigger concern is combat pace and mechanics, I dont want to list here all common concerns because others already did, but i agree that the current combat design kind of forces the player to try generic bonus actions with all party members in all rounds, which kinds of stealing the show from the unique actions and make the player to go for similar tactics for all battles and also, staying on topic this makes the battles much longer and tedious as well. I remember Swen said in a podcast that they tried out a lot of different variations of the action economy, multiple actions per turn (felt too allowing), original dnd5e economy (felt too limiting), he even mentioned at a certain point Bonus action was not even a thing. They went with the current system as a compromise (a tweaked bonus action system), im not sure if they counted in how much this can slow down combat and take away from tactical aspect of the game (especially long term). Im not against freedom of choices in combat (i do like that Larian likes to try out different, even radical things because these can turn out very well also) but the action economy need to be carefully balanced (and to be honest, at this point it seems to me raw dnd 5e rules did this balance better, as you can feel from Solasta combat gameplay) so its not turning to be repetitive for such a long game. I dont mind if here and there unique use of these general bonus actions reward the player but they should not be so easy to use and cheese fights with in order to keep them interesting. Im in the camp who is waiting for an alternative, official dnd game mode for this very reason.
Eventually, if they improve things like we mention here, i think combat won't feel slow and less repetitive, the game already has superb-looking but at the same time pretty fast animations (much more faster than Solasta, where i feel animations are too slow in general), which really improves the experience.
All in all, they say a lot that EA is for testing how satisfied people are with current features/mechanics and not for bugfixes, so hopefully beyond fixing AI and UI issues they will listen and provide some alternative options for combat rules. Fingers crossed.