Here's maybe a useful chart on kayak speeds/efficiencies: http://www.kayakshops.com/kayak_speed.htm
The data is not current to 2009, but it makes for interesting reading. You can click on any listed speed and the boats will rearrange themselves. There was an article by a marine architect in a 2008 issue of SK where he notes that every kayak will have one or more "sweet spots" where one gets the most boat speed for the least effort, and he shows you how to figure this out for your own boat. Like Magooch, I'm in no rush (most of the time) while I'm paddling; I paddle at a speed where I feel I'm gliding smoothly along using a relaxed, unhurried, easy stroke. Makes for a great day on the water.