Every source I've been able to locate suggests that the delay in ticks feature added to command blocks is meant to be a delay in redstone ticks (1/10th of a second), but in reality the delay is in game ticks (1/20th of a second).

To reproduce, place an impulse block with

tell @a start

follow with a chain with a delay in ticks value of 300

tell @a end

Use a stopwatch to record the time between the start and end messages. You'll find that the time elapsed is 15 seconds, when it should be 30.