how much is missed out depends very much on the speed of car as the missed section is just a flat out straight and you normally back off straight after the gantry to cool down before coming off. i would say something like between 20 to 40 seconds but have never driven the full lap to judge it.

i timed mine with gps recorder and a camcorder, as camcorder only footage can easily be sped up by 5% or whatever without you being able to tell. not that i'm suggesting anybody has.