Yes, hard water affect performance of heater.
Hard water results in formation of scale deposits when heated. The result will be reduction of heat transfer through the layers of scale; and water will take more time to heat and the element can also fail earlier than it rated life.
Set the temperature to a lower setting below 50°c, scale forms will be more when water is heated above 50°c. Reducing the temperature will reduce the scale formation and avoid problems of energy wastage and element failure.