Ironically, simplifying the combats to more closely follow the rules of 5e would (presumably) greatly speed up combat for the AI. Rather than having to cycle through multiple possible uses of their Action/BA (Shove, random scroll in their inventory, grenades, some special homebrew attack, etc.) the overwhelming majority of 'large fights' would involve low CR creatures that just have 1 or 2 possible Actions to choose from (usually 1 melee attack or 1 ranged attack). One of the beautiful aspects of the 5e system is how much each little part (monster abilities, player abilities, etc.) combine together to create something amazing. The whole is greater than the parts, etc.
Just go look at a 'large' fights in the BG 1 and BG 2 series. RTwP vs Turn-based aside, the overwhelming majority of large fights involved very simple enemies. It gives the player a chance to show off and feel character growth when 20 enemies burst through a door, only to have the formerly frail Wizard launch a Fireball in their faces and abruptly end combat. That sense of character growth and moments to reflect on how much stronger you are now than you used to be is lost in BG 3 right now. Not every fight has to be unique with new spells or abilities. Sometimes enemies are just there because it makes RP sense for them to be there, not just to show off some flashy new gimmick.
+5 to this, not every enemy needs some new homebrew ability, special arrows, grenades, triple attacks, etc. Making the enemies closer to their actual 5e counterparts with only 1 or 2 actions/bonus actions available to them would be fine, and I'm sure there are plenty of ways to do that without even making the enemies feel weaker. For instance, the gnolls. Currently in the game, gnolls have some stupidly insane triple attack action (I'm sure everyone has seen my hate rants for them by now lol), and the higher CR variants of them have even more abilities it seems. Simply moving them closer to their 5e stats would make it less taxing on the AI because they would have 3 attacks to choose from: melee weapon, ranged weapon, or bite. Replace the ability to move and bite attack if you drop a creature to 0 hp with a simple bonus attack bite so they can make 2 attacks a turn (action and bonus action bite), remove the triple attack, and remove some of the random homebrew abilities and special arrows they all seem to have.
Fights don't need to have a million different things going on and contain tons of new abilities and flashy explosions to be exciting and fun. Just let me fireball some goblins and have a feeling of being stronger than them without having to worry that 3 of them are gonna shoot acid arrows or start casting cleric spells out of nowhere. If every enemy encounter in the game has some kind of caster, every archer has special arrows, and every other enemy has a grenade of some kind, it feels less fun and just gets bogged down.
The reason goblins and gnolls are low level early game enemies is because they're supposed to be relatively easy to fight enemies that aren't terribly OP compared to a new party, and are a good way to get used to combat. If introducing low level enemies with triple attacks is supposed to be getting me used to the way combat works in this game, I'm quitting now before I discover a Lich who can make 5 attacks per turn or some crazy BS.
