just count and average as you go.

$setTimeStart = time;
$setCount = 0;
while (<IN>) {
   # process line here
   
   $setCount++;

   # show line speed for every 1000 lines so prints don't
   # slow you down so much
   if($setCount < 999) {
      $setCount = 0;
      $setTime = time - $setTimeStart;
      print "processing 1000 lines at " . $setTime/1000 . " per line\n
+";
      $setTimeStart = time;
   }
}
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