APC: http://pecl.php.net/APC

Benchmarking PHP-5.4.5-dev w/ APC-3.1.11-dev --enable-apc-pthreadrwlocks
~> siege -c 5 -b -t30s "http://wp1.localhost/?p=1"
** SIEGE 2.70
** Preparing 5 concurrent users for battle.
The server is now under siege...
Lifting the server siege..      done.
Transactions:		        2259 hits
Availability:		      100.00 %
Elapsed time:		       29.70 secs
Data transferred:	       18.24 MB
Response time:		        0.07 secs
Transaction rate:	       76.06 trans/sec
Throughput:		        0.61 MB/sec
Concurrency:		        4.99
Successful transactions:        2259
Failed transactions:	           0
Longest transaction:	        0.13
Shortest transaction:	        0.03
Make sure we are cpu-bound
~> vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 7  0 297704 333808 103644 680204    0    0     0    32  278 1682 90 10  0  0
 5  0 297704 333184 103644 680348    0    0     0     0  279 1668 94  6  0  0
 6  0 297704 333516 103644 680500    0    0     0     0  269 1701 92  8  0  0
 6  0 297704 333076 103644 680612    0    0     0     0  266 1651 93  8  0  0
15  0 297704 333780 103644 680640    0    0     0     0  274 1718 93  7  0  0
 6  0 297704 332248 103652 681548    0    0     0    32  328 2519 86 14  0  0
 5  0 297704 329704 103652 684272    0    0     0     0  286 1623 90 11  0  0
 7  0 297704 330208 103652 684184    0    0     0     0  374 1793 93  7  0  0
 7  0 297704 332452 103652 681164    0    0     0     0  268 1678 95  6  0  0
 7  0 297704 332196 103652 681460    0    0     0     0  291 1667 93  7  0  0
 6  0 297704 332336 103660 681484    0    0     0    20  278 1718 92  8  0  0
 6  0 297704 333412 103660 680792    0    0     0     0  279 1655 91  9  0  0
The cpu idle column is 0, and the io wait is 0. If you have id>10 and wa>10 then you are likely io-bound and you need to track down your io issues instead of worrying about PHP.