My experience so far is in order to accomplish any a bit more sophisticated in terms of email content, you need a custom shortcode. Although the custom shortcodes seem to work in terms of displaying content, I am starting to observe that they seems to affect loading/sending time significantly.

For example, my custom shortcodes pull some content from an API, then store it in a transient in the database. MailPoet is able to pull the content from the transient, but it takes forever to finally display it... Hence, when the emails are sent it seems that each email is generated separately, i.e. each time MailPoet 'produces' an email to each of the thousands of subscribers it calls the database to get the content. Is this assumption true? I would have thought that once an email is queued for sending, it is converted into HTML and sent to all subscribers as static content.

Does anyone have similar experience?