What is the relation between pressure and airflow speed above an airfoil?
Above the wing, decreased pressure causes the air to accelerate. But it is the plane's velocity that causes the decrease in pressure above the wing.

Much like pressure gradients are the cause of wind.

Or when a fast truck on a highway punches a hole in the air, the air behind the truck rushes in to the lower pressure area.

So when a fast truck/airfoil causes low pressure, the air rushes in / speeds up.


  Bernoulli's principle states that within a steady airflow of constant energy, when the air flows through a region of lower pressure it speeds up and vice versa.


Source: Wikipedia

More information here (NASA).

What happens in a stall, is the drag from the airflow separation outweighs the lift. But the pressure is still lower above the wing.

Leading-edge root extension and delta wings can make better use of high angles of attack.

The NASA link above explains the true relation between airflow speed and pressure, but that's not the only cause of lift, is what I mean.