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.
