Perhaps a moderator will move this thread out of ENGINES and into OEM + Custom Interiors.
The USUAL problem with poor air flow is not related to the blower motor unless the blower motor is defective.
The main issue is typically a debris-plugged evaporator core. Pine needles, leaves, cottonwood fluff, butterflies...anything that falls into the HVAC plenum can be pulled down into the blower motor fan area, and then plastered against the Evaporator.
NOT the worst I've seen...but the only one I have photos of.
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The second issue is a lack of voltage/amperage (power) to the motor due to high resistance in the circuit, or a failed blower motor.
At minimum, a two-step voltage-drop test of the blower high-speed circuit is warranted. Test the + side of the circuit--battery + to motor +; and then the ground-side--battery - to the blower motor ground tab. You may have to extend the leads of your voltmeter using primary wire; the gauge size is fairly unimportant. Anything heavier than 16-gauge is plenty, 18 gauge may work too.
Ideally, you'd use a low-ampere current probe and an oscilloscope to check average amperage draw, and check each individual bar of the blower motor for current draw, looking for one or more bars that differ significantly from the others. And from there, you can calculate the motor RPM.
LASTLY, the quality of the motor itself is something to assess. I put in a "top quality" HVAC blower motor from NAPA into my '88 K1500. When it died some years later, I got an OEM motor from ACDelco. The Delco motor was obviously heavier/more powerful than the so-called "top quality" NAPA motor.
Other possibilities that are less common: Fan "squirrel cage" slipping on the motor shaft--fan turns slower than the motor. Debris inside of the fan, or in the plenum blocking air flow.