All email was like this at one time. In the 90s we were told to expect emails to take a few hours.

If you want immediate response use IRC or actually phone someone.

If was like this because it was designed before networks were expected to be fully connected at any one time. Mail would hop its way towards its destination as intermediate nodes dialed in over modem to upstreams.

It was like that because each check/send process was pop3 call that looked more like processing a queue of work for offline clients.

Now it’s more like being subscribed and listening for events.

The transition from POP to IMAP was a big step forward, back in the day.

I had email at my house, in the early 90's, over a UUCP feed. It polled every 4 hours.

I still manually check for mail. I don't want distracting notifications popping up at random.

Think about it in context. Polling constantly would consume a non-trivial amount of both bandwidth and server resources. 15 second intervals would require 60 times as many very expensive servers to handle a certain number of users as 15 minutes would.

And in the 1990s, people were just adopting email, and their existing context was mail that arrived daily. Other than IRC, nobody had ever experienced any instant instantaneous form of communication other than phone calls, so nobody really felt like they were missing anything if they didn’t get notified about an email for 15 minutes.

Today by contrast, I feel like 3/4 of the time I go into my email client it’s to get some login code sent by some stupid website, so from this perspective 15 minutes sounds like absolute torture.

This seemed a little hard to believe but it sounded like it was more of a replication where it was downloading things into an offline mode where you then write your emails and send them back again period. Other email clients might do this as well if you're running it on your desktop.