What Is Turbo Lag? What Is Boost Threshold?
Honeywell (the makers of Garrett Turbochargers) defines boost threshold as: "Boost threshold is the engine speed at which there is sufficient exhaust gas flow to generate positive manifold pressure, or boost."
They also define turbo lag as: "Turbo lag is the time delay of boost response after the throttle is opened when operating above the boost threshold engine speed. Turbo lag is determined by many factors, including turbo size relative to engine size, the state of tuning of the engine, the inertia of the turbo's rotating group, turbine efficiency, intake plumbing losses, exhaust backpressure, etc."