Certainly! Here's an updated version of the function that includes a performance test:

``` 
function count_weekend_days($start_date, $end_date) {
$start = new DateTime($start_date);
$end = new DateTime($end_date);
$interval = DateInterval::createFromDateString('1 day');
$period = new DatePeriod($start, $interval, $end);
$count = 0;
foreach ($period as $date) {
if ($date->format('N') >= 6) {
$count++;
}
}
return $count;
}

function test_count_weekend_days($start_date, $end_date) {
$start_time = microtime(true);
$result = count_weekend_days($start_date, $end_date);
$end_time = microtime(true);
$time_taken = $end_time - $start_time;
echo "Result: $result\n";
echo "Time taken: $time_taken seconds\n";
}
``` 
The performance test function, test_count_weekend_days(), takes in the same arguments as count_weekend_days() and returns the result of that function as well as the time taken to execute it in seconds.
\
As for which metric to use, both time taken and complexity can be useful depending on the situation. Time taken gives a more concrete idea of how long the function takes to run in real-world scenarios, while complexity can give insight into how the function scales with larger inputs. In this case, since the input is limited to a date range, time taken is likely the more appropriate metric to use.
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