Sometimes, a fan is very speedy because the voltage of electricity it is getting is very high. When the voltage becomes low, the fan slows down. But most of the times, electricity or voltage is not the reason behind the fan speed. The way a fan operates depends on its manufacturer, style, and era in which it was produced. Fans made by some companies usually have a higher speed than others. Different fan manufacturers use different speed controls during manufacturing. If your fan does not slow down, it must probably not be having a high ranged speed control. It is merely the way it is designed, so you should not think of it as a problem to be fixed.