Anyone below Master Rank feared the venom of a yellow snake, but most Awakened with Elliot's stats wouldn't have much trouble enduring it for a certain period.
Although this venom doesn't work as well on Awakened as it does on others, it is possible to increase its effects. The venom attacks blood cells, especially red blood cells, and it has been shown that it can travel through white blood cells to spread faster.
This is why it can kill so quickly.
So, what would happen if the density of the poison were increased? At higher densities, its speed remains the same, but its effect is amplified to the point that it can immobilize the Awakened and, given enough time, kill them.
That's why Elliot's movement was so slow. He had so much poison concentrated in his arm that, even though his power prevented the poison from moving quickly, it could still immobilize him.
The situation, which was already desperate, suddenly reached a new turning point, where, given enough time, he would die.
