A radiator works best if the water inside it slows down and lingers to emit the heat into the room. Flow speed depends on the cross section of the pipe and the volume per time being pushed through, a radiator is equivalent to a much wider pipe section.
A massive airbubble in the radiator decreases the effective cross section of the radiator. meaning that less heat transfer happens in that radiator and that you are returning hotter water back to the boiler which means more heat loss from the return pipe.
Another factor is that you lose quite a bit of surface area that could be used to transfer the heat into the room.