I personally don't think the difference amounts to a hill of beans for 95% of paddlers.
I've done a lot of paddling with a GPS and heart rate monitor to try and get an idea of the speed and efficiency of many different boats I've owned. I've also been tinkering around with hull design and using Winters' Kaper software that plots speed vs. resistance. Yes, in general, making a boat longer makes it a bit faster on the top end. And, in general, taking off 6" to 1' of length gives less resistance at low/medium speeds. But these differences, in my opinion, are slight.
The speed advantages of a long hull don't really come into play until you get above power levels most paddlers don't want to maintain for more than a couple minutes. And the efficiency advantages at low speeds offered by a shorter hull are somewhat negated by the fact that it takes such little effort to maintain those speeds anyway.
The padder is what really makes the difference when it comes to speed although it's an unpopular answer and one I try to ignore myself, always searching for the elusive magic canoe.
More important, IMO, is how the hull shape fits with your paddling style. A long straight keeled hull might feel slow to a single sided paddler when it doesn't want to respond effortlessly to correction strokes and a shorter, more maneuverable hull, might feel slow to a hit and switch paddler when it constantly wants to yaw back and forth requiring correction strokes.
Alan