Hey, thanks for phpList! We were on it for the longest time, and personally, I first used it some 14 years ago.
We did try several things; Postfix tweaks, SES / Postal SMTPs, rate changes in the phpList config etc., but nothing seemed to work. This, along with the other factors (segmentation, dynamic templating), lead to listmonk.
Not sure what SMTP setup you had with phpList, but sending 1.5m messages on a VPS should take less than a day with the right Postfix tweaks.