I’ve noticed on mine that if the center vents on the dash are open the defrost vents have very little air coming out of them. If the windshield starts to fog I just close the center dash vents and push all the air out the defrost vents. As for the lower vents, I thought they had pretty decent air coming out of them.
I think it depends a lot on how much vent line you used. I cut any and all excess vent line making it a shorter run from the heater core to the vent.