Redmine

Do you have a lot of users in your instance ? From my experience it can take a while to build the notification recipients list (which is calculated synchronously). Please try to confirm that, we have some pending patches that could improve this in the next versions.

Probably because you've configured email mechanism and it is using smtp / sendmail instead of async smtp or async sendmail? Sending emails is a length process specially if multiple users are supposed to receive notifications about the issue update / creation.