How does the middleware::terminate() method work?

I thought that if I create a middleware with a terminate method, its content will be executed after the response is already sent back to the client. The following experiment disproved it:

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Http\Request;
use Illuminate\Support\Facades\Log;
use Symfony\Component\HttpFoundation\Response;

class QueueEmailsAfterResponseIsSent
{
    /**
     * Handle an incoming request.
     *
     * @param  \Closure(\Illuminate\Http\Request): (\Symfony\Component\HttpFoundation\Response)  $next
     */
    public function handle(Request $request, Closure $next): Response
    {
        return $next($request);
    }
    
    /**
     * Handle tasks after the response has been sent to the browser.
     */
    public function terminate(Request $request, Response $response): void
    {
        sleep(15);
        Log::info('Works after response is sent');
    }
}

So I have to wait 15 seconds before the web browser is refreshed (and logs the message). But I thought that the appropriate behavior would be that the browser refreshes immediately, and then I would have to wait 15 seconds to see the newly created message in the log file.

I have read the Symfony HttpKerlen component's documentation, from which Laravel implements its terminate() functionality (so I am not really interested in that part).  ...or maybe I have not paid enough attention?

Inside the Laravel's doc, it sas:

If you define a terminate method on your middleware and your web server is using FastCGI...

If I use the built-in server using the php artisan serve command, then the phpinfo() shows these lines:

CGI / FastCGI : Rasmus Lerdorf, Stig Bakken, Shane Caraveo, Dmitry Stogov FastCGI Process Manager : Andrei Nigmatulin, dreamcat4, Antony Dovgal, Jerome Loyet

... it is enough evidence for me, that FastCGI is used by the default dev server, isn't it?

So do I misunderstand the terminate() method, or does it work but not with the sleep(15) function? As you can see, I would use it to unload the current request-response lifecycle for sending multiple emails. Will it do its job?