Your right many things can cause low air flow. I visually inspected the fan and it is obviously turning on slow speed. I could try to manually by pass the board and try running it on the black wire to confirm that the fan is turning slow.
Model # DGHA075AD148BN
Serial # D893231057.
I said " 1989 Snyder General HVAC unit that we only use for AC" . I'm sorry I thought that would be clear. It is not just an air handler it is a furnace.
We have put up with it for years. I decided this year to look into it.
Thank You