Mailer Going Very Slow

[Hershel]

I have a Drupal 7 CiviCRM 4.4.5 site with cron set to run every minute and on civicrm/admin/mail?reset=1 we have

Mailer Batch Limit   100
Mailer Throttle Time   0
Mailer Job Size   0
Mailer CRON job limit   0

yet only around 10 emails are going out every minute. Any ideas why this might be ?

[Hershel]

The site owner clarified now that this slow behavior is only with one mailer with "long and complex content." Other mailers shoot out at 100 a minute, as expected.

Seeing as how this is an unusual situation, it may not warrant further investigation.