The FIFO only works if the average input rate no greater than the average output rate.1
In this case, the output is continuous, so the average output rate is equal to the output clock rate.
If the input is bursty, then then average input rate is the input clock frequency multiplied by the burst duty cycle.
But if the input is continuous, then the average input rate is equal to the input clock rate, and this violates the first rule stated above. The FIFO will overflow.
1 Sometimes flow control is used to insure that this is true.