An http_load run
5000 fetches, 5 max parallel, 5.1645e+07 bytes, in 74.0229 seconds
10329 mean bytes/connection
67.5466 fetches/sec, 697689 bytes/sec
msecs/connect: 0.219601 mean, 1.106 max, 0.171 min
msecs/first-response: 72.1792 mean, 1209.1 max, 14.354 min
HTTP response codes:
  code 200 -- 5000
Whenever you do any sort of load testing, you need look beyond just the raw numbers and have a look at what your server is actually doing. Use vmstat:

Base PHP Load
6:33am riddler:~> vmstat 1
 procs      memory      page                    disks     faults      cpu
 r b w     avm    fre  flt  re  pi  po  fr  sr ad0 ad2   in   sy  cs us sy id
 5 1 0  175880  59292  491   0   0   0 408   0   0   0  858 7725 128 90 10  0
 5 1 0  175844  59296  481   0   0   0 414   0   0   6  869 7714 133 85 15  0
 5 1 0  175880  59256  492   0   0   0 414   0   0   0  873 7798 114 82 18  0
 5 1 0  175880  59248  472   0   0   0 402   0   2   1  848 7599 146 89 11  0
 5 1 0  175880  59232  479   0   0   0 408   0   0   0  858 7649 130 89 11  0
 5 1 0  176232  59244  479   0   0   0 414   0   1   0  863 7673 144 82 18  0
 5 1 0  171444  59256  790   0   0   0 672   0   3   0  826 7770 158 91  9  0
 5 1 0  171480  59216  478   0   0   0 402   0   0   0  843 7626 139 89 11  0
 5 1 0  171444  59224  480   0   0   0 414   0   0   0  862 7673 124 90 10  0
 5 1 0  171516  59168  491   0   0   0 408   0   0   0  846 7699 147 87 13  0
Our benchmark test was deliberately designed to have quite a bit of PHP processing and not a whole lot of output. 10k is somewhat small for a web page. If instead we have a whole lot of output, chances are we will be io-bound instead of cpu-bound. If you are io-bound, there is little sense in optimizing at the PHP level.

Evidence of an io-bound test
 procs      memory      page                    disks     faults      cpu
 r b w     avm    fre  flt  re  pi  po  fr  sr ad0 ac0   in   sy  cs us sy id
 4 0 0  132860  15724 1033   0   0   0   0   0   0   0 4457  954 3704  2 25 74
 5 0 0  132860  15724 1009   0   0   0   0   0   0   0 4436  714 3597  3 24 73
 6 0 0  132860  15716  980   0   0   0   0   0   0   0 4446  925 3603  5 23 72
 2 0 0  132860  15716 1028   0   0   0   0   0   6   0 4514  720 3696  2 24 73
 3 0 0  132472  15716 1018   0   0   0   0   0   2   0 4501  946 3673  2 22 76
 4 0 0  132472  15716 1039   0   0   0   0   0   0   0 4565  737 3718  2 26 73
 3 0 0  132472  15708 1010   0   0   0   0   0   2   0 4498  938 3639  2 24 75
 2 0 0  132472  15708 1012   0   0   0   0   0   0   0 4543  730 3665  5 25 70
Things to try if you are io-bound
[php.ini]
output_handler = ob_gzhandler

[Apache-1 httpd.conf]
LoadModule gzip_module lib/apache/mod_gzip.so

[Apache-2 httpd.conf]
LoadModule deflate_module lib/apache/mod_deflate.so