To help with this, please see this worked example for End of Page delivery: eop-example.
What other things can you do to help speed?
Caching is important. OpenX has built-in caching which limits calls to the database – in a production environment you might want to increase the cache time used by your installation. You might also want to install a caching system for PHP on your server.
Keep in mind what 3rd parties you are using for banners. If you are using external image banners or 3rd party HTML banners, not only must OpenX deliver the banner but the 3rd party must then deliver the image/HTML. Sometimes such external sources can be slow to load and should be kept in mind when analyzing speed.