Okay I have a 2013 crew. My fan has blown slow since I purchased my truck 2 years ago. I realized the recirculating blend door was broken as is common. I removed the dash and replaced the blend door and actuator. One everything was back together I tried it and it now can switch back and forth between fresh air and recirculating. You can hear the fan speed pick up but it’s still really slow even on high. I compared it to another 2013 f150 and mine on high is compared to the other medium. So I replaced the motor speed resistor and that didn’t help. I also checked the plug for the resistor and it looks fine. Next I changed the fan motor itself and there was no change. Next I cleaned the evaporator on both sides and that didn’t help any either. Please help. I don’t know what else to do.

 

Does that model year have a cabin filter -is it clogged?

I suspect you would have seen any debris if it exists, when you changed the blower motor.
One common cause of slow speed HVAC air discharge is from a critter making a nest in there. You should see the mess though if you've been in there I think.

 

F150s don’t have cabin air filters prior to 2015. And I cleaned inside when I had the fan removed