If the blower motor is physically slowing, that's either due to a wiring issue (including the blower motor resistor), a charging system issue, or a problem with the blower motor. I suppose there's a slight chance there could be a restriction in the inlet air path that causes drag on the blower motor when the vehicle is not moving, but that would be one of the last areas to check, or you could easily switch between "Fresh/Norm" and "Recirculate/Max A/C" to change the inlet path to see if it has an effect.
-Rod