I usually use mine immediatly, and it's been rock solid consistent. I would advise not keeping it out though, the less dust exposure the better. When I'm done using it, it goes right back in it's foam padded box it came in.
As far as speed.....after it dispenses a charge (and you pour it into a case) you put the empty powder tray back on the scale, which it quickly re- zeros and starts to dispense again. By the time Im done seating the bullet and inspecting the round, it's ready.
BUT, I do check every weight on the display screen before I grab the tray. Big cylinder shape powders ( h4350, retumbo, etc.) is harder to trickle and get exact. There is a sensitivity setting on the unit to accommodate this. You also can place a straw on the trickle port ( great idea, not mine, somebody on this forum) which is supposed to even the flow when the unit slows down and starts to trickle the powder near the end of the pre-set charge. Or Just set the unit about .1 -.4 grains light then just LIGHTLY tap the powder spout, which makes the powder come out one granule at a time, to the desired charge.
Ball powders or smaller granule powders ( ramshot magnum, h380, etc.) are exact every time.