pss includes a queue that limits the amount of message sends to be kept in memory. Currently channels are used to manage this queue.

Sends from the queue are handled synchronically. This is a potential bottleneck which has been proven by @nonsense already with stress testing.

To allow asynchronous sends, it must be guaranteed that the message can be re-inserted in the send queue if the send should fail. The current implementation doesn't allow for that, since the queue theoretically can fill up while the message is being attempted sent.