In the Simplenews docs there is the following tip:
The Elysia Cron module (http://drupal.org/project/elysia_cron) can be used to start the simplenews cron hook more often than others, so that newsletter are sent faster without decreasing site performance due to long-running cron hooks.
This is great because it will clear the mail spool without waiting for several cron runs to occur. However what are the performance implications of running this cron task on an ongoing basis with a short interval of say 30 minutes? Especially if a newsletter only gets sent once a month. i.e. My simplenews cron task in running every 30 minutes with nothing to do.