Since when does airspeed affect a turbines acceleration rate? It might in a sr-71 when operating as a ramjet, but not in engine with an operating compressor which 99.99 percent of engines have, unless its a rocket. The lag is due to the fact that you have to accelerate a mass from 20,000 to 90,000 rpm without overtemping the hot side, which is the primary limitation for any turbine. Fuel metering is digital fuel system based off of N1 speed (compressor) and the turbine outlet tenprature and combustor pressure used as inputs and limiting factors. most turbines are slow in the low end, but have very fast accelation rates in thier operating rates, so i doubt the delay is much over half a second or so. 