With transfers generally taking 2 days to process, this allows you time to put the award on hold before transferring the points. (upgradedpoints.com)
Advantages of using AA miles include being able to book one way awards, put the award on hold for up to 5 days before ticketing, and having AA call you back if there's a long hold time. (travelsort.com)
Amex MR transfers tend to be instant but you may want to consider putting the award on hold if possible before transferring. (themilesgenie.com)