Actually the answer is that, technically speaking, tap water will take slightly longer to boil than pure water, but in reality the difference between the two will be negligible. Tap water will contain dissolved ions and chemicals in the water, including some added by municipal water systems like potassium fluoride (KF) for tooth enamel or chlorine to kill microbes. The presence of these ions and chemicals will have the physical effect of extending the liquid temperature range of water. You may have heard phrases like "melting point depression" and "boiling point elevation." Adding salt to roads in the winter helps lower the freezing point of water so that ice and snow will melt even on a cold morning. In a similar way, the ions and chemicals in tap water will make it boil slightly higher than 100 degrees Celsius which will be the exact boiling point of pure water. So since the tap water boils at a slightly higher temperature, it will take slightly longer to reach its boiling point versus pure water. As I said earlier, the difference in the boiling points between the two will be minimal, less than 1 degree Celsius.