[Hampshire] Any PHP/ZEND Experts?

Hi all.

Just migrated a clients PHP application using ZEND framework from a 2 CPU
Fedora 8 server to an 8 CPU CentOS 5.4 one (apart from Zend pretty much
standard LAMP), similar story with backend database server except more
memory. Both new servers have pretty much negligible load on right now, the
frontpage loads in a flash, but as soon as PHP/Zend clunks into action it
goes horribly slow. I've never used Zend, any pointers or obvious gotchas I
should look for?

I usually install APC with PHP installations, I've been told by the
developers though that they're already using Zend Optimiser - not sure
whether the 2 can live together, or whether it's worth turning off the Zend
one and using APC instead.

TiA