Capt Turbo's post hit the nail. These calculations nearly always have to be done in the five minutes before departure as that's when the load Figures come, and in some airlines the performance computer is also the electronic tech log, which is grasped in the engineer's oily hand until then. There are usually dispatchers, cabin crew, push-back crew, engineers, "on-time managers" (really!) and ATC all competing for attention so the flight deck is anything but a place of quiet concentration.