There are a few reasons :
1) There are bugs. The AI takes forever to choose what to do, sometimes the ennemies are frozen or stuck etc... They'll work on that.
2) We don't have enough action/round. 1 more character in our party mean 1 more action + 1 more bonus action (+1 reaction) / round.
Too much time watching instead of playing.
3) There are way too many creatures in some combats. Fighting the goblin camp (inside but especially outside) and/or the druids and tieffelin is a pain.
4) The balance of the game is really bad and every encounters are designed arround this terrible balance.
- Goblins have more HP than they should
- Minotaurs or gnolls have more attack/round than they should leading to the die/help/die/help loop
- If you don't use dipping with a candle you'll loose lots of additionnal damages to kill ennemie's faster (+1D4/weapon + burning damages)
- If you don't use shove/thunderwave you have nothing to easy kill any creatures
- If you don't use...
- ...
Combats can be a bit faster with all cheeses and you can avoid many misses but they become easy and not interresting (using the same "tactics" at every combats whatever your party composition).
5) Ennemies never use dash
6) The concentration mechanic is broken and ennemies have too much surfaces potions, meaning you'll waste lots of turns casting and re-casting the same spells over and over.
7) All combats are designed to be "challenging" and none is designed for you to feel a bit powerfull (i.e trash combats against wolfs in the forest)
8) The ennemy can't deal with verticality, meaning they'll often attack you with disadvantage, wasting their turns (and our time). They should tone down those bonuses so it's not happening "nothing" anymore.
9) Reactions are broken, meaning you'll use hellish rebuke to kill a goblin with 2HP instead of OS another full life one.
I probably forget some reasons.
Point 7 is a really important one that I think is overlooked by most game designers these days but it overlooks a critical aspect of game design. I made a whole topic just about this a while back. But every one of your points is excellent. It's also succinct enough that it should be easy enough for Larian to address it. I really hope they do.
