Relevant animals: greenland sharks. ~20 ft sharks slowly, slowly drifting at approximately 0.5 mph around the North Atlantic and killing time until they bump into some edible decomposing thing. They reach sexual maturity at about 150 and kick off around 400 [1].<p>[1] <a href="https://en.m.wikipedia.org/wiki/Greenland_shark" rel="nofollow">https://en.m.wikipedia.org/wiki/Greenland_shark</a>
In some science fiction movies (for example "Life", "The Green Slime") the alien life forms grow incredibly fast. If that works or is possible, it makes one wonder why no terrestrial life-form didn't adapt a similar strategy. In the above movies the fast life-form in question does come from a dead planet (the probable outcome of such strategy, but not really something evolution has enough time to select against).