Here is the thing about a prompter asking if you want to take a reaction, and if so; what that reaction will be: It *WONT* make things feel "slow" or stop the pace of the battle. Why not? Because it's on an enemy turn. It will just feel great because even though the enemy is doing things on their turn, suddenly stuff pauses and YOU get a say in the development of the enemy actions! It gives the player agency and control, and that wont feel like "the battle is slowing down" or anything like that, cause these things happen outside your turn.
I *TRULY* wish they implement something like that.