here is a little script :
<?
include "time.inc";

Begin_Timer();
for ($i=0;$i<63000;$i++)
{
  $string .= "a";
}
Show_Timer();

Begin_Timer();
echo $string;
Show_Timer();
?>

from an http client on the SAME machine the output is :
<!Elapsed time : 0.300685>
lots of a
<!Elapsed time : 0.000917>

Everything is normal

BUT if I get the page from another machine on the same (non busy) network the output is:
<!Elapsed time : 0.299803>
lots of a
<!Elapsed time : 3.021531>

I tried with output buffering on and off, same results.

doing a 
for ($i=0;$i<strlen($string);$i++)
  echo $string[$i];
is even faster.