She knew all of Sefiro's tricks.
Sefiro didn't expect an answer from "Catnine," and she murmured, "What poison did she use? How do I neutralize it? Why didn't she just kill me with a potent one? Why did she use this one?"
Feng Wu laughed inwardly when she saw how distressed Sefiro was.
The poison she gave Sefiro wasn't all that potent. It came from the tail of a Red Snake Scorpion, but she had refined it with her special technique, making it colorless and odorless, so Sefiro couldn't detect it.
As for why this poison…
Because Feng Wu wasn't going to let Sefiro die just yet. If Sefiro died, she could only do so after the race was over.
It was the scorpion's venom that was causing that itchy and prickly feeling to ensure that Sefiro wouldn't be able to reach the finish line before Feng Wu did.
She still wanted to take all of Sefiro's wealth, so of course she wouldn't let her win.