Email alerts are generated like this:
1) any monitor does tests periodically.
2) there is a value (numeric field) indicating the number of monitor fails when alert is triggered.
3) alerts, assigned to a monitor, are executed one by one.
4) at the same time, they may get a delay in email because of email service queue
It would be helpful to see your logs and to know if you're getting the same delay with other alerts (say, appending text-file and see modification date of the file). I would recommend opening a support ticket.