Large parts of the worldwide energy system are undergoing drastic changes at the moment. Two of these changes are the increasing share of intermittent generation technologies and the advent of the smart grid. A possible application of smart grids is demand response, i.e., the ability to influence and control power demand to match it with fluctuating generation. We present a heuristic approach to coordinate large amounts of time-flexible loads in a smart grid with the aim of peak shaving with a focus on algorithmic efficiency. A practical evaluation shows that our approach scales to large instances and produces results that come close to optimality.