IMHO, if you want to mass email your forum members it would be better to not do it through Discourse.
If your intent is to mass email, as compared to mass Notification, I think it would be better to export your members’ email addresses and use an email provider.
As for a possible delay, it could be that a job needs to run. But it could be that some members, depending on their activity and how they have their Preference Settings that they won’t get an email.