There are several factors that can influence how long it will take for an email to be delivered.
Including list size, server activity, and ESP throttling (this is when the ESP decides how often emails are delivered to recipients.) If you have chosen to add an attachment via the Communications feature, this will greatly delay your email.
On a busy day, your email will be placed in queue and sent as the queue clears up. Check our Communications Dashboard for real-time updates on all deliveries.