I'm just going to add how quick and smooth combat is in Solasta. The game has pop-up reactions AND combat is faster than BG3.
The logic that pop-up reactions may slow down the pace of combat, is not true at all.
Originally Posted by CJMPinger
Their engine should definitely be able to handle a reaction system as it'd mostly just be adding a pause and then allowing player to to choose something. I think it'd be less having to remake things and more just adding to what is already there. But this might be too generalizing or completely and utterly wrong.
The enemy can use reactions too. So it'd be a structure similar to...
add a reaction table for the round (reactions are bound to the round, not the turn). It'd be similar to maintaining initiative order.
when character uses reaction ability, reaction point is spent
for each character with initiative, reaction point refreshes on start of turn
create conditional classes/statuses to prompt use of reactions
use those classes to pause combat (player side) and prompt the player
create combat AI for enemies that may use reactions (probably always will, unless out of spell slots, out of reactions for the round).
Of course more needed code would come out of the woodwork when implementing/optimizing. I think it's totally doable as well.