The problem is, that with the new Email-API the software puts each email in a queue (which you mentioned). Every 10th request (from a visitor) the software perform one action in the queue (e.g. send one email).
If you have 1.000 users you need 10.000 page visits until the queue is empty (after a newsletter). And there are many more tasks which can be queued (packageserver request, email for activation) and this tasks are queued at the end of the queue, so that these actions are executed after the newsletter. For email activation, the waiting time is very bad.