I have an Alfine dynamo front hub, and I don't notice it at all. Of course, I have a rack, pannier, studded tires, saddle bag, lights, and an upright commuter, so it's not like I put it on a 12 lb carbon bike. The drag caused by running the dynamo all the time is probably less than half a stoplight--my precision measurement for performance impacts-- the effect of the added weight is less than that. For comparison, the studded tires are 5-10 stoplights.