For optimal chick quality and hatching results, we want to have a short hatch window (time between the first and last chick hatching) with the chicks hatching on time, to assure that they are not longer than necessary in the hatchers.
We see sometimes that the hatching process takes very long, and that especially the last chicks are coming very slow, sometimes even so slow that they are not in time to escape from the egg shell before the hatch is pulled, resulting in a reduced hatchability.
If the hatch is slow (a "dragging" hatch), we usually see more chick quality issues related to navel problems, and then especially with the appearance of string navels (a piece of dried membrane that is trapped in the closing navel, appearing as a string from the navel).
The main reason for this dragging hatch is a too low temperature for the last chicks hatching. During the hatching process the chick needs to express a lot of activity, and the hard work of escaping from the egg shell results in an increase in egg shell temperature, usually to a level of approximately 102-103oF. If the temperature in the egg remains too low, the chick will not be that active and will not escape quickly from the egg shell.
Not every machine is the same and can be set in the same way, but we usually see that the hatch becomes slow and "dragging" if the ventilation of the machines is increased too early during the hatching process. We want to increase the ventilation to keep the carbon dioxide in the machines on a reasonable level and create a comfortable environment for the chicks that have already hatched, but if we increase the ventilation too early we will introduce more cold air in the machine, force to machine to spray water to keep the humidity up, and with it create too much coolling for the last chicks hatching.
Again, not all machines are equal, but if possible we prefer to keep the ventilation of the machines on a limited level until we see that the relative humidity in the machine has peaked, indicating that the last chicks have hatched. However, if the cooling capacity of the ventilation of the machine is not sufficient to keep the temperature under control we might be forced to increase the ventilation earlier, as overheating is a very dangerous situation for day old chicks.