The reason you use a poison that takes a long time to kill when you just need someone dead by a certain time (such as, perhaps, before an underaged king takes power some time in the future-and remember, the impetus for most events in the series is that Maekjong was underaged, and is only just now reaching an age where he can rule without a regent, so there was no urgency on either side of things for years) is that if you’re poisoning someone really prominent (like, say, the Queen Regent) and they suddenly get sick and die everyone gets suspicious, but if they slowly get sicker and sicker over a period of time, it’s easier to make it look like they just got sick and couldn’t get better, and it also helps to delegitimize them because they were sick and maybe not thinking right for however long it was.
Sidenote: I know for a fact that most people complaining about this watched Moon Lovers, so be sure to include “just like when Moon Lovers had Wang Mu slowly poisoned via his bathwater to drive him mad and delegitimize him to make it easier for Wang Yo to take over” when lodging the “who uses a poison that takes forever to kill someone” complaint.
(Not saying it’s a brilliant plot point, but neither show is the first to use it, or the last. Goblin used it to, for another recent example, as have other sageuks and ancient cdramas I’ve watched over the last decade or so. Not even limited to asian dramas,really.)
We throw around the word never likes its nothing but a small rose petal. The word never is a doubled edged sword. On one side it says ‘I will never leave you’ and on the other it says ‘I will never love you.’