There needs to be a balance between speed and player engagement. If everything is automated, that's boring. Yay. It's fast, but I've quit tablet games in minutes because their combat was way too auto. It's not fun if I'm not able to make smart choices and I'm just watching the game play itself.
But it is also frustrating when you're just sitting there waiting for your turn watching NPCs act or even other players. So, yes. It can slow combat down terribly if you don't do something to speed up OTHER characters' turns.
And multiplayer makes it even more essential for fast combat. Have you ever had to wait for an indecisive player? It's like Hell has captured you and put you in an Infernal Machine. This is why Reactions COULD get bad if done with pure prompts.
Imagine you're playing with another person who is a spellcaster. Enemy casts Magic Missile. Prompt for the other player so for you combat is paused. You wait. "Hey. What you doing?"
"Can't decide whether to Shield spell or not.". You urge then to move on, so they say No. It happens again. Enemy melee attacks mage. Pause combat with prompt. You wait. They can't decide whether to Shield spell or not.
Yeah. It could make gameplay very painful.
