Benchmark
% http_load -parallel 5 -fetches 100 ~/drupal.txt
100 fetches, 5 max parallel, 634100 bytes, in 25.5106 seconds
6341 mean bytes/connection
3.91994 fetches/sec, 24856.3 bytes/sec
msecs/connect: 3.95143 mean, 34.813 max, 1.935 min
msecs/first-response: 1242.85 mean, 25450.3 max, 234.405 min
HTTP response codes:
  code 200 -- 100
Let's ask Callgrind
% valgrind --tool=callgrind --dump-instr=yes -v /usr/sbin/apache -X
...
% kcachegrind drupal_mysqli1.out
And XDebug
[php.ini]
zend_extension=/usr/local/lib/php/extensions/xdebug.so
xdebug.profiler_enable=1
xdebug.profiler_aggregate = On
xdebug.profiler_output_dir=/tmp
...

% kcachegrind drupal_xdebug1.out
After
1000 fetches, 5 max parallel, 6.452e+06 bytes, in 12.2807 seconds
6452 mean bytes/connection
81.4287 fetches/sec, 525378 bytes/sec
msecs/connect: 4.79381 mean, 86.273 max, 1.9 min
msecs/first-response: 50.7257 mean, 129.51 max, 15.991 min
HTTP response codes:
  code 200 -- 1000