JBoss Portal community project is not currently in active development. The full force is on GateIn Portal (http://www.jboss.org/gatein/) so that's something to keep in mind when you start your work.
If you need to use JBP for whatever reason, you should at least use 2.7.2 which has several fixes found in 2.6.* versions. The supported version of JBP is Enterprise Portal Platform (EPP 4.3) and I believe lots of these perf problems are either solved or support folks will give you tips on how to do that.