Fins definitely allow you to swim faster. How much speed improvement you'll experience will depend on how well you use the fins. Some snorkelers and swimmers hold their feet and fins to high and they are out of the water for a good time of the kick. At that time you obviously won't get any speed improvement. However, if you use good technique for your kicks and leverage the increased size to propel you forward then you can easily see improvements of 30% or even more.