Genetic drift can be rapid, but this is usually associated with some stochastic event - like if due to rapid climate change (a la "Day After Tomorrow") all there was to eat was rice and beans for a long time, a lot of people (like me) might well bloat-up and die before they could reproduce much. But of course there would be other selection pressures involved in such an event that might compensate, who knows.

But sometimes I wonder if the rising incidence of juvenile obesity and diabetes is just a slow sort of selection... these poor kids appear to all have the genetic tendency towards IR, and they are at risk of being weeded out of the gene pool fairly early nowadays. It will be awful if the trend continues.