I replaced my bathroom exhaust fan switch with a countdown switch

I replaced the switch of the AirKing bathroom exhaust fan with a countdown timer.

However, even the timer is working as expected, I noticed a drop in the fan power, it does not blow as much air as it used to blow. So, I think the issue may be related with the output of the countdown timer. The manual (see below) mentions an output of 500W. But, I am not sure what is the wattage of the fan. Would you agree that this is the reason for the drop of power? Can you recommend me what should I look for when buying the right countdown timer for a exhaust fan?