According to the research, the first crack is caused by high pressure steam escaping the bean. Water escapes before the first crack, as the bean dries, but in liquid/vapor form; until the cell walls crack, the pressure is so high inside them, that steam only forms around 350F.

If the coffee is past crop, very dry and with cracked cell walls, the first crack won't happen at all. On the M3, open the trap door when the bean temp thermometer reads 150C and sniff -- a wet grassy haze should be coming off the beans. If it's dry and smoky, you have past crop beans.

The first crack will be weak and late if the coffee is dry, even if it is new. Naturals and pulp naturals tend to have quieter and later first cracks -- whether this is the processing itself or longer delays in shipping, I don't know.

As far as I can tell, a weak first crack doesn't usually mean you can improve the roast by going faster. If the coffee is past crop, it may even hurt; since old coffees need to be roasted at lower temperatures. If the coffee is not past crop, the usual indicator of a too slow roast, a flat, dull taste, is more diagnostic than the cracks.