Find out where your PHP code is slowing down (Performance Issue)
I've used XDebug profiling recently in a similiar situation. It outputs a full profile report that can be read with many common profiling apps ( Can't give you a list though, I just used the one that came with slackware ).
As Juan mentioned, xDebug is excellent. If you're on Windows, WinCacheGrind will let you look over the reports.
Watch this presentation by Rasmus Lerdorf (creator of PHP). He goes into some good examples of testing PHP speed and what to look for as well as some internals that can slow things down. XDebug is one tool he uses. He also makes a very solid point about knowing what performance cost you're getting into with frameworks.
Video:http://www.archive.org/details/simple_is_hard
Slides (since it's hard to see on the video):http://talks.php.net/show/drupal08/1