If you think a piece of PHP code is inefficient but don't know where, there is a BlueBox function that can give you time-splits as the code progresses.
bb_timer([optional __LINE__ number],[optional return_value]);
The simplest use would be bb_timer(__LINE__); which will print to the screen the time taken to get to that specific line of code.
If you would rather have the value returned then enter a 'true' value into the second value.
