How much user drop-off is caused by slow email delivery in your app?

Turns out password reset emails taking almost a minute to arrive was causing a huge chunk of our user frustration.

People would get impatient, hit reset three times and give up or flood our support inbox. Replacing email links with a quick on-screen verification code for active sessions fixed most of those tickets instantly.

Are there any parts of your app where delayed transactional emails are quietly killing the user experience?