I'm not sure if the issue is on my side (mail servers are gmail's) or on the mailing list server - all traces I've checked indicate that the delay is somewhere in the delivery from postgres' last mail server to the first gmail mail server.
I've only really noticed this sometime in the past few weeks. After sampling my mails, I found other examples of significant delays (>1h) for mails from well-respected hackers dating back to at least 2023-08-28.
I have noticed the same thing happening for the Gmail account that I use.