I can't be fucked to find the graph but it's true. The relationship between speed and exhaustion is highly asymmetric, going 20 % slower than your default walk is much more tiring than going 20 % faster.

It actually makes intuitive sense to anyone who hikes. Walking slow is like hiking downhill. You're fighting your body's natural cadence which requires extra energy that doesn't go anywhere useful. Going faster also uses extra energy but it's largely "useful" energy that spares steps. The extreme version of that line of thought is that I can run 10 km but not crawl 10 km.

Also fast walkers tend to... walk a lot, it's a virtuous circle. It's fine if grandma does a lap around the mall at 2 km/h, but I've been walking all day and I have more places to walk to afterwards and I ain't doing all that at crawling pace.