From what you describe, it sounds like you have a thermostat stuck open and are operating the climate control in the Auto mode. If you have the blower control in the auto position, blower speed is dependent on coolant temp and heat demand. (Also vehicle speed but that's a bit more in depth than is necessary here) If you are demanding heat but the coolant is still cool, you won't get much blower airflow. Once the coolant heats up, the climate control system computer automatically (hence the "auto" mode name) increases blower speed. Once the heat demand is satisfied, the blower should slow down again.

A new thermostat is cheap and easy, if it were my car I'd try that first....

J. M. van Swaay