Ok, I found a nasty killer query:

Can't remember. Does it show anywhere ?

However, I have replaced half of the queries by now and added some additional code.

I am going to replace all queries that take longe then 0.001 seconds though.

Maybe I'll check out JavaBB some day.

p.s.: the last query was not phpbb's fault however, they do not make use of inner joins and often you find "joins" using where statements.
Also the topic view counter should be in a separate table to avoid locking during updates that occur almost every second...therefore I made a cron job to update them only once an hour for the time being.

The problem with PhpBB is the permissions tables...for every single users you have an additional rows in some tables.....so if you have over 300,000 users you have lots of unnecessary data in your tables....

The most information given out in the source code is version 2.0
No info on the minor release and I've not had reason to look at v3

Unfortunately the way to track down problems is accounting which puts more load on the server! Looks like you're getting there though. I tend to do my best not to modify the base code unless I have to. Makes it easier to upgrade to a less buggy version in the future