If your vehicle is showing weak airflow from vents even on high fan speed, here are the most likely causes and the parts you'll need to fix each one.
The cabin air filter traps dust, pollen, and debris before air enters the interior. If neglected, it becomes completely blocked, choking off the airflow from the blower motor.
If the cabin filter was recently serviced, the plastic shipping wrapper may have been left on, or the filter was installed backwards against the directional airflow arrows, severely restricting air.
The mode door directs air to the correct vents (dash, floor, defrost). If the actuator fails, the door can get stuck halfway or default to the defrost vents, making airflow from the main dash vents feel extremely weak.
The blower motor fan (squirrel cage) might be packed with leaves, debris, or rodent nests. Alternatively, the motor's internal bearings or brushes are failing, preventing it from reaching its true maximum RPM.
If the vehicle was driven without a cabin filter, dirt can permanently clog the evaporator fins. Alternatively, a faulty temperature sensor can cause the condensation on the core to freeze into a solid block of ice, blocking all air until it thaws.