What is 'brake lag' in the context of air brakes, and roughly how long does it take?

AAMVA CDL Manual, Section 5, subsection 5.4.4 (Stopping Distance), pp. 5-8 to 5-9

Air brakes don't respond the instant you push the pedal the way a car's hydraulic brakes do, and that half-second-plus delay translates directly into extra feet traveled before the brakes even start slowing the truck. Underestimating this lag is a common reason new CDL drivers follow too closely or misjudge how much room they need.