I'd imagine they'd move faster, but suffer from internal crystallization, muscle spasms, accelerated decrepitation of the brain and olfactory senses, short-term memory loss, (it stops in the middle of a chase because it forgot it spotted you) etc. Really, I doubt you'd have to worry about crack zombies, even if they were L4D special-types. They'd probably rot to death sooner than a normal one and be far less effective in the horde. Hell, they'd probably be a tripping hazard to the rest of the zombies.
PROTIP: If you get bit and can't kill yourself, snort some crack. Hell, snort ALL the crack. Carry 6 bricks of crack with you at all times in case of emergency. If you can't make magazine armor like in WWZ, cover yourself with big bags of crack. For the good of humanity.