Why my thermostat is cycling on and off immediately?
Q:  Why cycling on and off?  Air is not blowing out with the same speed as before.

A:  Insufficient airflow at the furnace is causing the high temperature limit to trip.

One potential cause for this is an obstruction, such as a clogged filter or partly closed seasonal damper.  Another potential cause is that the blower motor is pumping less airflow than it should, possibly due to a failed run capacitor or worn bearings.

On most furnaces the high limit will not self-reset in a short period of time like the OP describes.  Instead, the furnace will lock out with an error code or trip a switch that requires a manual reset.  The high limit control needs to be carefully inspected to verify it is being tripped and to ensure that it has not been compromised in some way.  If in doubt, higher a professional.