Less airflow over the coil will cause a few things to happen.
1. The temperature rise over the coil will be higher because less heat is being carried away from the coil.
2. The higher temperature rise is less efficient, especially with a heat pump system because the higher the temperature difference, the more wattage the compressor is pulling. Non-condensing furnaces are also less efficient because more heat is lost up the flue since less is being captured. Condensing systems still lose some efficiency, but not as much. With condensing furnaces, if the air flow gets cut off too much, it will no longer reach a condensing temperature which will cause the efficiency to plummet and run the risk of melting the plastic exhaust pipe, there are temperature limit switches in place designed to prevent that from happening.
3. The lower airflow means less heat transferred to the living space, keep in mind that as temp rise over the coil increases, the heat pump loses capacity as that temperature rise occurs. CFM*(temp rise*1.08)=BTU output.
4. Defrost cycles will increase with the increased runtime and temperature rise.
So basically your system is putting out less heat over time, running longer, and consuming significantly more power for the same amount of total heat needed in the process. Heat pump systems need the amount of airflow from the install specifications to get their rated efficiency and output. The same applies to air conditioning too, without the proper airflow temperature drop is increased, capacity and efficiency are also lost and there is a risk of the indoor coil freezing and liquid returning to the condenser and slugging the compressor.
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