This is an old Bernoulli explanation of how wings work that is miss-leading or just plain wrong. The air is not moving, the wing is. So to answer your question, since the air is not moving.. it can't slow down.
As the wing moves forward, the air is compressed under the wing and decompressed above it. That difference in pressure causes lift. The angle of attack changes the relative amounts of each till you reach a point where the air becomes turbulent above the wing. At that point the lift becomes erratic and drag goes through the roof.
A simple analogy can be demonstrated by taking sand or talcum powder and sprinkling it over a table or sheet of cardboard or whatever. Then take a cardboard cut-out of the cross section of a wing and sliding it flat across the sand in the normal wing like direction. The sand under the wing will clump together and form a line of sand along the bottom side of the wing. The sand above the wing will be moved away from the wing leaving a void above the wing.
Now, of course, air is not like sand, and hates a vacuum, and will rush back in to fill the void above the wing. However in order to do so it is "stretched" forming low pressure above the wing. Similarly the air under the wing tries to get out of the way and equalize itself, but in the meantime it is compressed and has a higher pressure.
It is this difference in pressure that causes a net force up through the wing to provide lift.