I found this script which should work:
<!-- put this at the top of the page -->
<?php
$mtime = microtime();
$mtime = explode(" ",$mtime);
$mtime = $mtime[1] + $mtime[0];
$starttime = $mtime;
;?>
<!-- put other code and html in here -->
<!-- put this code at the bottom of the page -->
<?php
$mtime = microtime();
$mtime = explode(" ",$mtime);
$mtime = $mtime[1] + $mtime[0];
$endtime = $mtime;
$totaltime = ($endtime - $starttime);
echo "This page was created in ".$totaltime." seconds";
;?>
This would time the whole page, so if you want to be really precise for the SQL statement then you could put the two blocks of code either side of your DB code. I havent tried it out so I don't know to how many decimal places is reports so you may need to use the round function in php.