Performance Issue Posting Invoices

We are using Dynamics 2009, and are developing some modules to handle our sales order processing. We encountered a performance issue with posting sales invoices. In order to rule out our developments as being the cause, we created a 600 line invoice in the demo company (un-modded) using demo data. Using the standard invoicing this is taking around two minutes to post. Would this be what one would expect?

Our server has 4 x CPU @2.16Ghz, and 12 GB RAM. Looking at the performance monitor it is using one CPU and that is being maxed out.

Any help appreciated

Nino