I don't remember any specifics about Slow, but there are a lot of different potions. Check if any of them adress Slow. Though from what I remember most debuffs just go away on their own. If nothing else helps you can always go back to town to ask the priest for healing.

at high lv whenever you open a trap door or cache, you get hit with slow debuff. I tried as many potion I could but I still can't figure out which one works. It irritating when you need to walk slow for 120 seconds.

Maybe an item granting Slow Immunity could help a bit. Theil Brechtold from Hillbrandt is currently offering me an "Assault amulet of Slow Immunity" that grants +10% to Slow Immunity. Try checking some merchants specializing in magic items and rerolling their shopping offer a few times until you find something of Slow Immunity there. Or you could also drink a few potions of Speed after you get slowed (a Bad Potion gives me temporarily +1 Speed while a Perfect Potion gives me +4 Speed (and a Miraculous Potion might give +5, I guess)). But it does seem there is no potion to cure Slow, judging from the Book of Magic...