Ever tried logging Apache page serve times using '%D'? You'll have discovered
that they aren't a good index of your server's performance, because they depend
more on the client's connection speed, computer and browsing habits than on the
speed of your server.

mod_log_firstbyte is a module for Apache 2.0 which allows you to log the time
between each request being read and the first byte of the response served.
Unlike the total serve time, this index of performance tells you how long Apache
actually spent loading the file off the disk or executing your script: it's
independent of client connection speed. It makes a great performance benchmark
for your server!

WWW: http://code.google.com/p/mod-log-firstbyte/