Just my 5 cents... I played Solasta a lot recently with a team of reaction-reliant classes (Paladin, Monk, Bard and a Wizard) just to see how i feel about their implementation of reactions and pop-ups.

I have to say i did not feel that the pop-ups are slowing down the game at all (I did feel the animations are really slow though compared to BG3 and that is massively slowing down combat for me), however they added a lot of tactical thinking to my playthrough which felt really good. I really liked their implementation overall, as others said it was also nice to be able to do things on the enemy's turn (it spiced up the generic my turn, their turn gameplay loop) - so im looking forward how BG3 will try to top or match this level.