Sheet cloning is time consuming about phpspreadsheet HOT 1 CLOSED

If you look more closely at the timings, you'll see that you're performing two actions, the clone and setting the worksheet title. And modifying your code slightly shows that the cost is in setting the worksheet title, not in the actual cloning.

    $t1 = microtime(true);
    $cloned = clone $template;
    $t2 = microtime(true);
    $cloned->setTitle('Sheet ' . $i);
    $t3 = microtime(true);
    echo ($t2 - $t1) * 1000 . " - " . ($t3 - $t2) * 1000 . "\n";

The reason for this is twofold: PhpSpreadsheet checks whether the worksheet name already exists (you can't have two worksheets with the same name). To do this, it has to check the title for every sheet that exists, and the more worksheets that you've created the more worksheets it has to check. Timing for this is fast enough that it shouldn't cause any problems.
It also checks every cell in the worksheet for any formulae that need changing, and that's where the performance overhead lies, because it also checks every cell in every worksheet.
You can disable this behaviour if it is unnecessary by passing an additional argument to the setTitle() call

$cloned->setTitle('Sheet ' . $i, false);

from phpspreadsheet.