This is a Quality of Life suggestion that in my opinion makes the game more polished than OG Maple Story had. With the addition of Monster Cards for example, you may get a card that is "+1 Speed while hunting in a certain area" & that will override Haste or a Speed Pill. If a player is hunting for monster cards, they may spend 15 minutes hunting 5 cards so normally they'd be on their 2nd spell pill, however they would have to use up 5 speed pills to keep up +10 speed as the +1 speed card more-so acts as a -9 speed "buff" in this case.
I'm suggesting this across all stats though, not just speed, I don't think a lower stat buff should override a higher stat buff just because it's used afterwards.