First of all you need to find out what is the reason for your performance issues: physical network, webserver, database.

For MySQL you could install phpMyAdmin and take a look at the Status page. This should give you a clue whether the database is the bottleneck.

For PHP you should examine the appropriate php.ini (guessing that you use Apache as webserver the php.ini can be found at /etc/php5/apache). The config file is quite self explanatory.

The most difficult part would be tracking down physical bottlenecks (cabling, switches, NICs, network traffic, etc). If you are not familiar with these things you should provide more information about your installation.