Assume that the flow at the level of the trachea is 500 milliliter per second, and that trachea diameter is 2 cm.
Now look at the bottom illustration. The vertical axis shows how surface area increases from the trachea to the smaller airways. Estimate gas speed at the alveolar level, say at generation 20.
According to the graph the surface area is about 1100 cm², the volume flow is 500 cm³ per second. Hence the velocity is 500/1100, or about 0.45 cm/s, or 4.5 mm/s This is about 0.003 times the speed at the level of the trachea.