In addition to what was said about them, proper reactions (like in Solasta) would actually speed up the game for one more reason: on each of your turns you don't have to think what might happen and which reactions to "turn on".
Simple example: Wizard with Shield and Counterspell and next in initiative:
1) spellcaster followed by Big melee creature that is running your way
2) little green goblin followed by spellcaster
In 1: you are afraid of being downed or immediately killed so on your turn you spend time to disable Counterspell that would eat your reaction on some minor spell
In 2: you absolutely don't care that gobbo is going to hit you for 6 dmg, so you disable Shield and enable Counterspell.
Once again: on every turn you have to stop and think which reactions might be useful to you and you will likely be wrong.
Proper reactions on the other hand are quick. Big hit I can Shield from? Click and done. Fireball coming in the direction of the entire party? Click and done.
Here is what they wrote several months ago:
While it’s not going to be in EA immediately, the features and mechanics that allow a character to perform an action as a reaction will trigger automatically. The players will be able to control which reactions they want to enable in anticipation of enemy actions. E.g. a wizard would disable their Attack of Opportunity but enable their Shield spell, which will be cast automatically whenever the wizard is targeted by an attack or Magic Missile spell.
