Question

Is it possible with JSF to make ajax calls that will execute simultaneously (not waiting for previous calls to finish before starting a new one)?

Answer 1

No, they are explicitly queued by specification, without any exception. See chapter 13.3.2 of the JSF 2 specification:

13.3.2 Ajax Request Queueing

All Ajax requests must be put into a client side request queue before they are sent to the
server to ensure Ajax requests are processed in the order they are sent. The request that has been waiting in the queue the
longest is the next request to be sent. After a request is sent, the Ajax request callback function must remove the request
from the queue (also known as dequeuing). If the request completed successfully, it must be removed from the queue. If
there was an error, the client must be notified, but the request must still be removed from the queue so the next request
can be sent. The next request (the oldest request in the queue) must be sent. Refer to the jsf.ajax.request
JavaScript documentation for more specifics about the Ajax request queue.

This is done so to ensure thread safety of among others the view scoped beans in the server side.