Configuring OpenX memcached will help you significantly speed up your banner delivery and i recommend you to use it instead of the default file File based cache.
OpenX memcached configuration from Web Interface
This tutorial has been made with openx 2.8.10 but should work on most of the 2.8.x installations.
First you need to go to plugin section and configure memcached servers. This can be tricky since there is no direct access from the web interface.
You need to change the end of the url with: /plugin-settings.php?group=oxMemcached ( for default installation you should get http://domain.com/www/admin/plugin-settings.php?group=oxMemcached )
Add your memcached server and port if it’s default installation on the same machine add localhost:11211 and expiry time of 600 seconds.
You can also use memcached with multiple instances.
Now you need to go in Configuration->Banner Delivery Settings and set Time Between Banner Cache Updates to 600 seconds and change Banner Delivery Cache Store Type to Memcached.
This can significantly improve the performance of your OpenX server, and your pageload will be much faster.
OpenX memcached configuration from command line
Also this can be done from the command line by changing this lines in you var/yourdomain.com.conf/php
[Delivery] cacheExpire = 600 cacheStorePlugin = "deliveryCacheStore:oxMemcached:oxMemcached" [OxMemcached] memcachedServers = "localhost:11211" memcachedExpireTime = 600
This is just one of the tricks you can do on your OpenX server, for more tuning check the Performance Tuning section of OpenX developer zone.