The heat pump will always produce air that is warmer than the temp in the house, but as the temp outside drops the temp of the air coming out of the vents also drops. So on a very cold day when the house temp is say 70F, the system might only be putting out air that's 75-80F. The air coming out of the vents doesn't really _feel_ warm and it may take an hour or two to raise the temperature in the house when you wake up or get home in the evening.

In my experience at least with relatively modern heat pumps (roughly 2000 and newer) it doesn't matter that much when outside temps are above freezing. But it quickly starts to become noticeable as temps drop into the 20s.