I thought it was possible to chain-whip a city down from, say, size 8 to size 1 within a single turn, but any individual whip cannot whip away more than half the population (I.E. you can whip 8->4->2->1 in one turn, but not 8->2->1 since size 8 is limited to a 4-pop whip at most). Of course you are still limited to finishing one production per turn, and you can't invest hammers into a production to avoid the hard/cold whip penalty without passing a turn, but it can be useful for large newly captured cities that would otherwise just starve down anyhow.