Ah I see. If you don't want users to use the feature until a certain point, it might be better to lock it off until that moment. At any rate I think it's fine having the slow-down feature -- I just think you need to know what experience you want to happen. If the player knows the slow-down feature, they are guaranteed to use it as it is fundamental to guaranteeing a successful play, and users don't want to replay a level if they have the option.
Great job!