I've replaced the basic email notification templates with more complex templates (with the same tags), and immediately noticed that performance degraded. Specifically, when I clicked "confirm reservation" with the basic email templates, this worked almost instantaneously. Similarly, it was less than a second between a click on the "cancel reservation" button and the message that appeared as a result. After I replaced the email templates, delay between clicking "confirm" and seeing "done" is like 5 or 7 seconds, and the delay between clicking "cancel" and seeing confirmation is 10+ seconds.