Why are jQuery AJAX request to Symfony Controllers are handled in parallel instead of async?
As soon as you start a session in php, php will lock it and subsequent requests will have to wait until the session is available again.
So if your symfony script uses sessions, you can only execute 1 request at a time using that session while the session is open.
Disabling sessions (if that even is an option...) or closing it when you don't need it any more will allow parallel requests.