Configure Server for optimal performance Apache/Plesk/CentOs

We are looking for someone to re-install configure our server for optimal performance. We have our own Virtual Private Server with the following specifications: Plesk version: 9.5.3 Linux [login to view URL] CentOS The website running on this server has very poor speed which wasn't the case before. We testing the same website on another server and it already performed much better. Also some features are not working like email. We can create email accounts but we can't use them (IMAP error) even though the DNS settings are correct. The site running on this server has been made in Joomla so any Joomla specific configurations are welcome. If you feel you have enough experience with working on servers please send me a motivation in a PM. Thanks in advance