Originally Posted by GM4Him
"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.
This would happen for most reaction systems, not just prompts. Aside from the fact that such a player would likely take much longer on their base turn than they'd spend on any reaction...

Current auto-reaction system: "I can't decide on which reactions I should toggle on/off this round."
Preset (target selection) system: "I can't decide which enemies to target for this round for reaction A. And reaction B. And maybe reaction C"
Overhauled reaction system: "I can't decide which (p)reaction I want to enact this turn."
Preset (out of combat) system: "Hold up guys, I need to fiddle with my conditions for enacting counterspell, AoO, etc. Don't enter dialogue/combat/explore without me."