Colin the surface drag extends out from the walls more than you would think, affecting the flow overall. One of the reasons dust collector flow claims are up to double of what they actually can move. They take the reading in the centre of the duct where the flow is the highest. Take readings across the duct and the average brings it down. Even the addition of a bell mouth entry can improve the airflow in a duct by several percent just because it smooths out the airflow at the inlet of the duct. Without it the sharp entry has a turbulent zone around the edge that reduces the flow into the duct. Think of it like the wing of an airplane. The smooth flow over the wing generates the lift to allow the aircraft to fly. Cover the wing with frost and the drag from those ice particles are enough that the aircraft can't fly or even get airborne. It all adds up especially in a small system where there is little difference between good collection and failing.
Pete
Edited to make the last line clear.
