Lets say you have a 4MHz clock and you set a delay of 1mS (1000us). A timer (without pre/post scaler) will count to 1000 for the duration of the delay.

Now drop the clock the 1MHz. The timer will now count to 250 (1000/4) each mS. However, the delay will increase to 4mS due to the slower clock, so the timer will again count to 1000.