I don't use PHPBB, so I'm not sure if this tip is helpful, but I've found on my forum software that disabling some of the unnecessary extras (things like user tracking, clicklogs, weekly statistics, etc) lightens the conuery load somewhat. I suppose if the software was very well written it wouldn't make much difference, but it did seem to for me.
On a somewhat unrelated note, I've always wondered how many conueries various software uses, but have never been able to make a good guess without installing it and letting users have at it--not good if it's too resource intensive.
So if you don't mind, for reference, could you tell us how many posts your board sees per day? It'd also be interesting if you knew how many of those users are active (ie posting every day or three) and/or about how many pages your BB is generating; total numbers aren't necessarily indicitive of traffic.
In the case of my YaBBSE forum, with an average of about 150 posts per day, 3000 pages generated per day, and probably four dozen active members, it's hovering around 4.5M conueries/month. That's out of 1200 registered members and 26K posts, though, so traffic is obviously a lot lower than the theoretical "size" numbers would indicate. I am wondering how this compares to other forum software, though.