Poison is one of the big nerfs to 5th edition. The poisoned condition RAW reads "A poisoned creature has disadvantage on attack rolls and ability checks." Poison in earlier editions was much truer to what poison should actually do to it's victims. If we take a D&D-like fantasy world, we are going to assume that most serious toxins delivered by what we might call "monsters" or giant sized animals are designed to immobilize or kill their opponents. While the 5e rule gets points for ease of use, it suffers as somewhat lackluster in describing the myriad types of toxins against which an adventuring character may be subjected. Smaller sized animals like tiny or even small spiders and biting insects are likely to cause minimal concern for most adventurers. Likely not even disadvantage on all attack rolls and ability checks. Snakes and some certain species of other venomous critters as one may find in our world, could indeed cause the poisoned condition, but very possibly death. 5e has simply removed that possibility from the game. But even real world statistics tell us that of the 5.4 million snakebites world wide about 2 million are envenomed and of those approximately 110,000 die from envenomation. Granted, we are dealing with adventurers and not common folk, but that is also expressed in higher ability scores, proficiency bonuses and better saving throw values.
Now let us consider large, giant or huge spiders and snakes. naturally their venom will be in larger quantities and designed to knock down or kill and jellify larger prey. Prey like adventurers. It just doesn't stand to reason that a single poison condition encompasses the effects of such venomous bites. So, I propose a more useful, "realistic" and somewhat more granular approach to poison in my 5e games. Something like the following is a start: