Think of it like a restaurant and you're at the restaurant with 3 others. The restaurant hands you a menu, and there are two pages of entrees to choose from. There's maybe a dozen total things to choose from, including sides.
Now imagine a restaurant with 5 pages of menu items. So many more entrees to choose from. Now there's maybe 24 to thirty total things to choose from.
Have you ever been out to eat with others at a restaurant with a ton of choices? There is always someone who cant figure out what they want. Narrow the choices, speed up the selection process. Too broad a menu, and you're likely going to have the waiter/waitress come back multiple times asking if you're ready to order yet.
The same is true for an RPG like D&D. If you start the game with a ton of options, and then you add more and more as you level up, multiplying the number of options considerably, before you know it, you're inundated with so many options, making decisions much harder.
When you reach higher levels, wizards alone will have TONS of spells to choose from. Add to that MORE options with scrolls and weapon special abilities and potions you can both drink and throw and shoving options and so on and so forth, and you're going to have players who are just sitting there very round wondering what they want to do, skimming their items and flipping their hotbar menus looking for what abilities they might want to use.
I'm thinking multiplayer in particular here, but single player as well. If I"m controlling 4 characters with a dozen or more options each every round, that's a TON of options to choose from, and the next thing you know, you're constantly skimming options as opposed to making quick decisions.
But aside from all this, having clearly defined and distinct classes, again, makes for a cohesive TEAM based game. Without distinct classes, with everyone having the ability to do everything, then nothing is special and unique. Likewise, if you have a ton of abilities right from the start, as you grow and increase in level, your new "special" abilities are no longer really "special" because everyone has so many "special" abilities immediately. So, what growth did your character really experience? Did they really gain anything truly special per level? Not really, because everyone can do everything.
Take clerics for example. In D&D 5e, only a cleric (paladin, etc.) can use a Revivify scroll. However, it's a Level 3 spell. That means that even a low level cleric might still fail to use a Revivify scroll. They have to make a roll to succeed in using it because it's above their pay grade. This alone makes a cleric a highly valued member of the party. It makes a cleric very special, and when the cleric finally reaches the level to be able to use a Revivify scroll without making a roll, you REALLY feel that they have become a powerful cleric. They've truly grown, and you feel that they've truly grown, because they now can use the scroll without a potential fail. That makes the journey so much more rewarding.
Right now, with BG3, everyone can use a Revivify scroll at level 1. So that completely strips the clerics (and like classes) from this very unique and special ability. No one feels that they've grown at all because they can use the scroll immediately without fail or issue. Everyone can use it. No one really grows or is special at all.
Basically, in BG3 right now, items trump classes altogether. As long as you find the right items, you don't need classes. You don't need spells and you don't need clerics and you don't need anything BUT items. Items give you special abilities and spells and area of effect and everything.
Items reign supreme in BG3. Classes and characters do not. And THAT is my whole point. There is no one who is special and there is no real character growth or development. Any that exist are minor and not very impactful at all. And again, combat is slowed down because you have so many options.
Here's another way to look at it. Current AI is quick, but it is still not super quick. The more options an enemy has per round, the longer the AI takes. We are at level 4 max. By the end of the game, each enemy has more options because they have better weapons and items. A single goblin might have the ability to Dash as a bonus, Disengage, fire a bow, swing a melee, throw a healing potion, throw a bomb, shoot a special arrow versus a normal one, shove, throw an alchemist's fire, some can cast a variety of spells, there are ranger goblins that can cast Hunter's Mark, lacerate, pommel, topple, etc.
So a single AI goblin can have a dozen options to choose from, and the AI has to decide for each goblin which move to make. In D&D 5e, a single goblin would likely have only two options, maybe three. Even if you homebrew a few, you might give them a few spells or maybe an extra move or two tops. You wouldn't give each goblin a dozen moves.
So why does it slow down combat? Not only do you, the player, have a bunch more options to choose from, you also have NPC's with a ton more options to choose from, slowing down the AI.
And it's only going to get worse as you increase in levels and gain more and more and more items.
