So, we have determined we are cpu-bound and we need to go faster. What can we do? Some low-hanging fruit:

include_path
include_path = "/usr/share/pear:."
<?php
    include './template_helpers.inc';
    include 'business_logic.inc';
?>
open_basedir
open_basedir = "/usr/share/htdocs/:/usr/share/pear/"
open_basedir checking adds some extra syscalls to every file operation. It can be useful, but it is rather expensive, so turn it off if you don't think you need it.

variables_order
variables_order = "GPC"
<?php
    echo $_SERVER['DOCUMENT_ROOT'];
    echo getenv('DOCUMENT_ROOT'); 
?>
If you never use cookies, turn those off too

Add an Opcode Cache

zend_extension=/usr/local/lib/php/20020429/apc.so
apc.enabled=1
apc.shm_segments=1
apc.optimization=0
apc.shm_size=30
apc.num_files_hint=10
apc.gc_ttl=10
apc.mmap_file_mask=/tmp/apc.XXXXXX
apc.filters=
Results
156 request/sec Standard PHP 4.3
161 requests/sec Fix include_path, and only populate GP
191 requests/sec Add IonCube Accelerator on top
196 requests/sec APC no optimizer, IPC shared mem + semaphore locking
198 requests/sec Turck MMCache no optimizer with spinlocks
200 requests/sec APC no optimizer, mmap mem + user space sleep locks
202 requests/sec Turck MMCache with optimizer and spinlocks