Real time web resource salt vs predefined (i.e. constant)

filemtime-clearstatscache.php

<?php

$mt = microtime(1);
for ($i = 0; $i < 10000; ++$i) {
    clearstatcache();
    $file = 'file-prefix.' . date('Ymdh', filemtime('some-file.txt')) . '.txt';
}
echo microtime(1) - $mt, PHP_EOL, $file, PHP_EOL;

predefined-salt.php

<?php

$mt = microtime(1);
define('WEB_RESOURCES_SALT', '2012051505');
for ($i = 0; $i < 10000; ++$i) {
    $file = 'file-prefix.' . WEB_RESOURCES_SALT . '.txt';
}
echo microtime(1) - $mt, PHP_EOL, $file, PHP_EOL;

some-file.txt

/* nothing here; file required by filemtime-clearstatscache.php  */