By default, when using HTML e-mail notifications format, Jira will hold the notification for issue updates for about 3~4 minutes (batching), whereas text format sends notifications for every update.

It would be nice to have an option to disable batching and deliver messages whenever an update is made for HTML format.

Workaround

If you go to Settings > System > General Settings and update the value of "E-mail from" to something different than its default value - "${fullname} (Jira)" - e-mails will no longer be batched but will affect every user.