He'll slack off for the recharging turn, so either way you'll lose just one turn. He won't recharge on his second move, he'll be able to attack again. Slaking is one of the few Pokemon that can effectively use Giga Impact because of his ability.
Note that if you use a recharge move, Slaking won't be able to switch out during the turn he slacks off, which is not a usual effect of his ability. So there is still a small drawback.