For trying to go fast (esp. on flat water), stiffer is better, as less energy is being expended to flex the hull, either vertically or horizontally. That energy is lost, because when the hull un-flexes, it does no useful work.
In rough water, a little flex can help reduce jarring as well as noise. My SOF feels smoother and much quieter going through or over waves and wakes than my FG, kevlar/carbon and wood boats.
Baidarka were often made with several slip joints along the keel line to promote flex. It is believed to be a way of keeping the boat quiet while paddling through waves while hunting; the energy lost to flex is traded for stealth.
Kayak Deck Gear Bags
Touring Kayak Paddles
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