Stings of the irukandji box jellyfish are apparent immediately, and people have died in as little time as two minutes.

I am not sure if it is the most "powerful" but the sting from a box Jellyfish is often a death sentence....

What is the most lethal fast acting venom like how bad the effects are and how long it takes before they become apparent or show symptoms ive heard about dart frogs and russell viper venom and banana spiders but they just say effects not how fast acting they are they are just like unbearable pain paralisis stuff like not being able to beather but how long does it take for these symptoms to begin after the inital bite/infection or what ever would be used