My dryer produces heat but not nearly enough. Like, 4hr for a load of towels.

I have replaced the heating element assy, the thermal limit fuse and safety thermostat. Also, the heater thermostat (in the btm pan under the blower) and its control. I have studied your xlnt repair forum and checked voltage, continuity and resistance everywhere as advised. I can't find anything wrong electrically.
The next step was to verify proper air flow. My exhaust duct travels vertically up an in-wall shaft, through the attic and out a louvered roof vent. Total length is not more than 10ft. I have checked outflow at the vent and it seems okay though I'm uncertain of what the correct cfm would be.

Please advise. I'm out several hundred $$ so far and don't relish the thought of having to buy a new unit after all.
Thank you.