Callgrind to the rescue! 

% valgrind --tool=callgrind --dump-instr=yes -v /usr/sbin/apache -X
...
% kcachegrind drupal_mysqli1.out
Install APC
200 fetches, 5 max parallel, 1.2682e+06 bytes, in 8.97701 seconds
6341 mean bytes/connection
22.2791 fetches/sec, 141272 bytes/sec
msecs/connect: 2.56332 mean, 4.705 max, 1.937 min
msecs/first-response: 215.859 mean, 395.53 max, 51.845 min
HTTP response codes:
  code 200 -- 200
% valgrind --tool=callgrind --dump-instr=yes -v /usr/sbin/apache -X
...
% kcachegrind drupal_mysqli2.out