If a ball happens to roll over a crack, the speed decrease must only take effect after the ball has rolled a distance equal to its speed.

Distance can never be equal to speed even though both are scalar numbers which can be the same number. They are different dimensions/units related by another dimension/unit, time. The description can make much more sense if you specify that speed is distance-units-per-second. Then you would say "For every second the ball rolls, and for every crack over which it rolls, its speed decreases by 1 unit/second" and "the speed decrease must only take effect at the end of each second."