Well recently I had to change IcyPhoenix to phpBB3 consumed as many resources on the server and therefore I am already adapting everything, but there is something good that even I can not find, and is counting new messages, a feature that IcyPhoenix had before, then it to help me do or tell me what the MOD to implement in phpBB3.
We don't give support for phpBB3 – You'd have better chances asking on a phpBB support forum (like phpbb.com).
I'd have liked to track down your resources issues, though, I'm sure we can improve performance in several places.