PHP Guzzle7 Concurrent Requests Not Working - promise

Question

Hi I have been trying to develop a script which will basically stress test one of my API calls. I decided to use Guzzle and have been following this tutorial - http://docs.guzzlephp.org/en/stable/quickstart.html?highlight=getasync#concurrent-requests
I think I may have a miss understanding on how concurrent requests should work or I have an error somewhere in my code.
Below is my entire script, its quite simple but will hopefully do the job
use GuzzleHttp\Client;
use GuzzleHttp\Exception\RequestException;
use GuzzleHttp\Pool;
use GuzzleHttp\Psr7\Request;
use GuzzleHttp\Psr7\Response;
$client = new Client();
$start = time();
$requests = function ($total) use ($client) {
$uri = '';
for ($i = 0; $i < $total; $i++) {
yield function() use ($client, $uri) {
$bulk = [];
$count1 = 0;
while($count1 < 40) {
$bulk[] = json_decode('{}');
$count1++;
}
$payload = [
"drone_id" => 56,
"bulk" => $bulk
];
$jws = new \Gamegos\JWS\JWS();
$headers = array(
'alg' => 'HS256',
'typ' => 'JWT'
);
$key = '';
$token = $jws->encode($headers, $payload, $key);
$request = new Request('POST', $uri, $headers, $token);
return $client->sendAsync($request);
};
}
};
$pool = new Pool($client, $requests(2500), [
'concurrency' => 10,
]);
// Initiate the transfers and create a promise
$promise = $pool->promise();
// Force the pool of requests to complete.
$promise->wait();
echo 'Request took ' . (time() - $start) . ' seconds' . PHP_EOL;
My plan was to send concurrent requests to try and enhance the stress test on the API. I originally had the pool concurrency property set to 5 and the script took 85 seconds to run, I then decided to up this to 10 which in my head means 10 requests should be sent in parrellel reducing the total time by half but after running the script it still took 80 seconds to run.
Could someone please check my code since I believe I must have done something wrong or I have misunderstood how the pool concurrency works.