Sacrosanct
Legend
You do realize that there is no neutralize poison in 5E. Lesser restoration can end the poisoned condition. If you notice in the DMG there is also no poison with an onset time listed. Finally, many monsters depend on poison for their damage output. Without such damage being a real factor in a short term fight, the CR of these monsters will be way more than they are worth in XP.
If the poison damage is ongoing, that infers that the target is poisoned. And that means they also suffer disadvantage. Taking poison damage instantly and immediately, and then "Heyo! I'm perfectly fine and not affected anymore!" five seconds later doesn't work me lol. I don't know any poison in the world that acts that way.