If the result of any action causes emails to be delivered, then the email delivery takes place before the action is completed and control returned to the user. The rationale is that in case of any errors the user is notified right then and there. If however, your mail server is slow in delivering emails, you can turn on Threaded Delivery option on the Edit Settings page via Toolbar→Setup→Site. When this option is turned on, emails will be delivered in the background, thus improving performance for the end users. However the drawback is that the user does not get instant feedback about the status of the email.