Someone who's extraordinarily slow, maybe due to an overconsumption of narcotics.
A: Man, I don't drink wine on Thursdays.
B: It's Friday, you crack turtle.
A crack turtle is someone whose mental capabilities have gone slow due to the excessive use of substances.
Jerry: "Oh man, I just needed two hours to take my morning dump and brush my teeth."
Tom: "You're such a fucking crack turtle, Jerry. You should consider throwing the pipe away."