Joomla Virtuemart performance optimisation

We have performance issues on our ecommerce website, currently running Joomla 1.5.22 Virtuemart 1.1.8 on a virtual managed server. On earlier versions of Joomla & Virtuemart we ran comfortably on a shared server with a similar number of products and more users with no performance issues.

Our site currently has over 1,000 products and over 150 categories.

We require diagnosis and necessary optimisation of templates / css / javascript / php / mysql to improve performance in terms of page load speed and cpu usage.