presumo tu stia usando tutto questo:

function getmicrotime(){
list($usec, $sec) = explode(" ",microtime());
return ((float)$usec + (float)$sec);
}

$time_start = getmicrotime();

.
.
.
.
.
$time_end = getmicrotime();
$time = $time_end - $time_start;
$time=round($time,3);

echo "Pagina caricata in $time secondi.";