You don't mention the version of PostgreSQL you have installed (9.2 is faster than 9.1), or how much RAM you have, or how many total users you support, how many concurrent users you support, or what they do, or how performance is now - what about those details?

These are the pieces that affect performance of OpenERP:

• Disk hardware - very little you can do
• Operating System - no experience with FreeBSD
• Database - PostgreSQL - see below
• Application - Python - Unless you want to start profiling code and re-programming there isn't anything here you can do
• Python Interpreter - Doubful there is anything here you can do
• Web Server - CherryPy - A single Pentium M 1.6 Ghz with 1GB can support over 500 requests a second
• CPU - nothing you can do apart from upgrade
• Memory - nothing you can do apart from upgrade/increase

So your only real option is PostgreSQL.

See http://www.slideshare.net/openobject/tips-on-how-to-improve-the-performance-of-your-custom-modules-for-high-volumes-deployment-olivier-dony-open-erp