I just rediscovered PHPlist after many years and installed in on an intranet server for testing. I am currently processing messages at a rate of 14 an hour which is obviously painfully slow. I searched the forum but found nothing close to this. I am currently sending to two users. One message took 6 min.
I am running it on Ubuntu 16.04/Apache2.4/PHP 7.0/Sendmail 8?
Whereabouts did you see this time reported? phplist will send as fast as it can, so any delay is going to be elsewhere. The php mail() function invokes sendmail (you can see the actual command in phpinfo) so maybe looking at the logs of the mail server might be a start.
I forget exactly where but I think it was the summary after the batch was sent. Since I was one of the two recipients I can confirm it did actually take this long or longer.
I have not looked at the mail logs and I will however as I said before I have two other apps running on this same box that do not have this problem using this same protocol. The only conclusion I can reach is that it is this application and not the mail server.