Come on guys can't you do better than this? You are using some old technology by duplicating PostNuke in its current state. Inline HTML is so passé…ever here of real templates…much easier for non-programmers to change their themes without having to touch code. Not even a WSIWYG editor --just a plain old web form? Shame on you for replicating PostNuke's awful URLs which are not search engine friendly.
Memory caching like was described here can only occur if you're delivering the same pages to visitors. If a site is delivering customized data to different people tied to access levels your server won't be able to store all those pages with varying data in memory unless your site hardly has any pages or minimal access levels. These will either have to be cached on a disk or *shudder* a database. Granted PostNuke and many of the other CMS' might benefit with some caching, but you give a totally wrong slant that Java performs better in this area.
I know sites which have 100,000-200,000 visitors with several thousand on at any one time running an open source Php CMS albeit not Post Nuke. Their pages load in a couple seconds max not minutes and their CPU usage is very low. IMO PostNuke is not efficiently coded and was not a good choice to replicate in its current state.
Yahoo, NASA, and many others have successfully converted some of their non-critical functions over to Php and MySQL. I cannot believe your site is busier than these orgs. The only thing I can think of is your problems are caused because of PostNuke's inefficient code.
Php does have its limitations. You guys have mysteriously elected to code Java down to these limitations. By all means I didn't come here to slam your project. I was really expecting to see more out of a Java app then what you have done at least so far. Don't just duplicate a Php app, use the power of Java to enhance it and deliver more especially on the user interface.