Almost all the "Instant" toxins are aeresols or gasses. . . .the circulatory system just doesn't work fast enough to kill you in seconds. . .and contact poisons usually need to soak through a semi permeable layer of skin.
Most of the dangerous pre-chemical laboratory toxins are all venoms of one sort or another. . .and I cannot think of any reptile, arachnid or bug that can kill you that fast. (Even things like Sea Krait or Blowfish Venom require minutes to incapacitate or kill you.) I know people who were bitten by Copperheads, Rattlers, Black Widows and Brown Recluses and all of them survived to make it to a hospital and a source of antivenom. (Of them, only the rattlesnake bitten man would have died untreated, he ended up loosing two fingers and spending a month in the hospital. . .but he walked a mile to his truck, and drove 10+ miles to a hospital delerious and in excrutiating pain.) I'm sure our australian members must have some snake stories. . . .
Watch a national geographic with blowguns with treefrog venom used on monkeys or other arboreals. . .they run away after being shot, the hunters chase after them on the ground, and collect the body when the monkey falls out of the tree minutes later.
Animal sourced toxins are usually protien based and need to be kept in airtight containers or refrigerated or they degrade in hours.
Plant based toxins, like Cynide or Castor bean toxins keep better, also very nasty. . .but also take some time. . .the most rapid form of death coming from inhalation of powdered toxin. (The entire surface of your lung is a better entry to the circulatory system than any dagger wound.)
The only instant incapacitating, near instant death toxins are all relatively modern, laboratory creations. . .and any party carrying around a sealed jar of alchemically created VX gas is in for problems. . .how would you use it? You willing to risk accidental breakage? Once you use it, you'd need to evacuate the area until there was a lot of rainfall to wash the nasty away. . . .there's reasons why people don't tend to casually use compounds like that. . .powder or gas toxins that poisonous are dropped from planes they are so dangerous to the attacker. . .and a weapon coating toxin that deadly would likely have deadly dust effects. . .an assassin creaping around in a haz-mat moon suit?
Plenty of RPGs downplay poison because it's often contrary to fun, in how it strikes down the weak and the strong. . .but RM seems to handle it fairly well. . .and with the various luck or Con based RR methods, they offer a broad range of lethality. . .but I think the system has the speed of effect pretty close to reality.