Turbo lag is the waiting time between the turbo starting to make boost and turbo reaching full boost. On the new gen WRX Sti, full boost can come on at 4000+ rpm. But this is all dependant on how you look at it. I would guess this figure was obtained from a standing start, 1st gear.what is this turbo lag everyone speaks of? on a street car there is not REALLY noticeable lag b/c hello, it IS a STREET car after all.