The "It slows down the game" response to reactions being implemented properly is an utter fallacy. It doesn't slow down the game, at all. certainly not more than the game slows itself down with its combative UI, endless circling and leap-frogging combat mechanics and ponderous AI.
A prettier solution to "Pre-cast the spells and then have it pop up a message asking if you want to spend it" would be just plain and simply implementing reactions properly - so the prompt comes up when appropriate (such as shield only when it would be effective) and you can say yes or no or in the case of spell reactions, select the level to cast at, all with one click, in less than a second.
Remember that you only get one reaction a round - one - and *most* rounds, *most* characters won't end up using their reactions anyway because the right circumstance won't show up. It does not slow down play at all, and indeed, it slows down play MORE to have to pre-cast reaction spells, because players will more frequently have to debate over whether to burn a spell slot or not on something that might or might not happen that way... as opposed to them not having to worry about that and only making the call specifically when it's relevant.
