Centos – Postfix on Centos – mail slow to send

We have a VPS running Postfix 2.3.3 on Centos 5.3, and it seems to take a long time to send an email.
There is an app running on the server (drupal) that sends off a series of emails on submitting a form, and it seems to be taking a heck of a long time to send them – like maybe 30s to a minute for each one. Even just firing off mail from the command line has a similar sluggishness.

If I tail -f /var/log/maillog I can see each email go past – there's a big pause before the first and in between each subsequent email.

I have only the vaguest understanding of what I am doing here, so I am not sure what other information I can provide that might prove helpful. As far as I know we just have a fairly bog-standard install of centos,postfix and webmin running.

Any suggestions as to where to look and what to do while I'm looking there would be much appreciated.