I am using accounts-ui and email to a send a email on registration. When a user creates an account, the response time is very high; it shows as nothing happens for a few seconds.
When I disable the email, the registration is way faster.
Kadira shows a response time of more than 2 seconds, with 1.3s on “email”, 0.8 on DB.
It sounds like your email call is being made synchronously, and from within a portion of the account creation flow which is waiting on the result before proceeding and logging your user in. I would look into making that call asynchronously, and handling any errors within the callback / promise resolution.