We have FMSA 9 on the exact same equipment, with the maximum amount of RAM allowable, and yes we also experience extreme slowness when loading pages via PHP/XSLT CWP (Custom Web Publishing), especially in comparison to other DB server technologies, such as MySQL.
We also have a G5 and an Intel Core 2 Duo, also running FMSA 9, and even those too frequently suffer from terrible latency issues when retreiving PHP and XSLT content via CWP.
In fact, even with the CPU at minimal usage, and low disk I/O, very frequently FileMaker's own PHP test page will still hang inexplicably! "What is it waiting for?" we ask. Obviously this is a very serious issue.
In our experience, even a multi-machine deployment can suffer from this critical problem.
A restart of the machine solves the problem, but only for a short while.