Audio problem: While Faith is stabbing the demon in his apartment, we hear the sound of her knife going through, and him yelling in pain. However, the last sound of her stabbing him comes a little late: The knife isn't even touching him.
Revealing mistake: After Faith brings Angelus back, when she goes to punch him and he blocks, the stuntman switch is very obvious - he's noticeably taller, has different hair and doesn't even seem to be wearing any vampire make-up.
Answer: She has superpowers, ergo fast movement. The reason she outruns him so easily in "Touched" is that she stops trying to actually fight him and just dodges instead.
Shay