Hey there!
Thats a great question!
With regular chicks... I had a silkie hatch once very fast over a 1 hour period...from the time it peeped, shell cracked, until it totally came out of the shell.
But by far the fastest to hatch were the Coturnix Quail.... Literally, they were popping out ---like popcorn---as I sat there watching the incubator. I think 3 or 4 came out in the few minutes I sat there watching. It was incredible!
Take Care,
Bill.