... In 1.3, the output of a PHP script was buffered internally by Apache before being sent to the client, meaning that a smaller number of system calls were used to send the data. Currently in 2.0 the PHP filter module uses the non-buffering interface to the filter stack, resulting in a large number of system calls each writing small amounts of data. ...Visit http://www.apacheweek.com/issues/02-06-14.
The latest Apache Week discusses the abandoned 2.0.37 release, PHP performance issues in Apache 2.0, and links to a few articles.